You're in it.
Well that's convenient!
I've been rooting through the new classes and races and I really like a lot of the stuff you've done with them but there are a couple that seem like they could have balancing issues and I was wondering if they've been playtested, as well as a few lines which you might want to adjust the language on to avoid confusion or someone coming along and trying to break the game.
pg 29 - Sidhe racial - Invisible to Mortals: I really like the flavour on this, and not just because I would almost certainly play one of these myself. I'm wondering if the once/rest invis has been thoroughly tested, though. It is a potentially very powerful ability, especially since you can get it at lvl 1, and it would need to be to mitigate that hefty weakness to iron that they have, but I can just imagine this being abused. If nothing else, perhaps consider putting in a duration and Concentration qualifier similar to the Invisibility spell, just so you don't have people using it for an entire day.
pg 31 - Tortle racial - Natural Armour: I once again really like the flavour of the Tortles and I really like this ability. I am curious as to exactly which way you intended to go with the power. If you intended it to be similar to the monk and barbarian class bonuses, which set the AC equation a certain way and a therefore not stackable - then you might want to consider adjusting the language to something like "Your AC is equal to your dex mod + your proficiency". If you did intend for it to be additive, just tweak the language a little so that it is clear that it is. And if it is additive, then you get into some potential balancing issues: If someone is playing a Tortle barb or monk, for instance, or a shapeshifting Tortle druid. However, if it is an AC replacement, it seems like it really hurts some character concepts and classes that rely on heavy armour and, unless you prioritize Dex, makes fighters really unattractive to play because they'll have no way to compete with just a dude in some armour.
pg 34 - Dervish ability - Path of the Desert: Just like the tortle, I'm unclear as to whether this is an ac replacement or bonus, because you can bet I'd be multiclassing into monk if it's an additive bonus. I do like the added touch of proficiency with melee attacks but not actual monk damage.
pg 34 - Dervish ability - Step Through Sand: The more I look at this ability the less I like it from a mechanical viewpoint. Either you're not on desert terrain and it is completely useless, which is sort of lame for a lvl 6 class ability, or you are on desert terrain and you have between 1 and 5 free lvl 2 spell slots which are bonus actions and give you nearly unrivaled mobility on the field, which seems incredibly strong. The Warlock could recharge all of its uses of Misty Step on a short rest too, but it would have used its actual spell slots to cast the spell, thereby limiting other options. The Rogue gets Cunning Action at lvl 2, which does let it use Disengage as a bonus action as many times as it wants, but the rogue still has to use its move action to actually move away. Comparing to other Druid lvl 6 abilities, they get advantage on some limited saves and can move through bushes without leaving a trail, or get to ignore DR on monsters. I like the idea of it, and I can see where you're trying to go with the flavour, but I don't think this works at all as a lvl 6 ability. I think it should do something minor for you if you're not in a desert and something much less powerful if you are.
pg 34 - Dervish ability - Blade of the Efreet: Poor Dervish, I'm just picking on it today. Once again, I'm a little leery of an ability, even a lvl 10 one, giving free lvl 2 spell slots for a bonus action spell, and unlimited slots at that. I don't think in this case it necessarily breaks the game but it does seem powerful and might need playtesting. However, I do think you need to be specific in the language for this ability because Flame Blade is one of those spells that increases in damage based on the level of slot used, albeit not very quickly. Does it cast at base level, which I think is closer to being reasonable, or can you cast as a higher level spell?
pg 35 - Diplomat ability - Eidetic Mind: This is the reason to take this archetype, in my opinion. I don't think it needs changing, I just wanted to comment.
pg 40 - Horse Warrior ability - Flurry of Death: Great art on this archetype, by the way. It's one of my favourites. The ability itself is probably fine. It wouldn't do much in my campaigns because I houserule mounted combat a bit and it'd just end up being redundant, but I can't criticize you for me cheating. However, it just feels clunky the way it's worded. I know this is a work in progress, I just wanted to point that one out.
pg 47 - Spirit Shaman - This dude's hat. Now that is a hat!
pg 48 - Makai Toa - I'm assuming you meant to have a regional/racial restriction on this archetype because otherwise this is incredibly breakable, especially if the Tortle's natural armour is additive. If that were the case, with a decent stat roll or point buy I could have an AC of 31 by lvl 10 without any magical equipment and just a cheap wooded shield. Even without the Tortle's ability it only drops to 27 without compromising anything, and potentially gets even easier depending on the stats the other race choice might have. You are fortunate there aren't half-orcs otherwise they would be unholy terrors with this as well. I'd also like to point out that unless you get the feat there's no such thing as Charging in 5e, so Storm of Blows probably needs to be looked at, regardless of balance issues. Oh, and the class is way out of alphabetical order.
pg 49 - Treekeeper - I really like this archetype. It works very well, and I love the lvl 14 ability.
pg 52 - Feats - Weapon Master Series: I see a couple potential issues with these feats. One is balance. For instance, it might be well worth it for me as an assassin-rogue to take a single level of fighter - it won't affect my top end abilities too greatly to miss a level whereas now I can sneak attack with a d12 rapier. Another issue is it's not explicitly stated what comes after a d12, so you should probably include that somewhere. The Grand Master 2 Hand option is assuredly overpowered in conjunction with the Makai Toa, possibly on its own. Lastly, just wanted to make sure that the Save vs Stun effects were intended to only key off the character's Str and not Dex. Probably needs testing either way, but I bet there's a bunch of Dex based characters who would love it if it worked for them.
That's it for now. I'll have to read more later, though. Hopefully I'm not being too critical because I really am impressed so far!