Narrow Adaptation. Adapted to the narrow caves and revises these merfolk count as one size smaller then they actually are for the sake of space they can effectively work in or squeeze through. In addition you can move through the space of any creature that is your size or larger.

Dashed Movement. Used to places where everything is vigilant and competitive the coralfolk have adapted to dash in and out quick enough to avoid retaliation. As a bonus action you can move up to 10 feet, take another action, and move back as part of this same bonus action to where you started. This movement does not provoke attacks of opportunity. This movement ends if you receive any amount of forced movement and you can't take this bonus action if your speed is less then 10.

Poisonous Retaliation. Living in places where they are just as much prey as predator the coralfolk have ways of discouraging attacks. When you take damage from a creature within 5 feet of you you can use your reaction to deal poison damage to them. The poison damage is your proficiency bonus amount of d8. A creature that takes this damage must succeed on a constitution save with a DC equal to 8 + your proficiency bonus + your constitution modifier. On a failed save the target is poisoned until the end of it's next turn. You can use this ability once per short or long rest.