Natural Luminescence. Aphotic merfolk have an affinity for magic that produces light. You know the Light cantrip, you can cast it as a bonus action and it doesn't require spell components when you cast it.
Starting at 3rd level, you can cast the spell Color Spray as a spell and when you cast it with this feature you cast it at a spell level equal to your total level divided by two. Starting at 5th level, you can also cast the spell Blur. Once you cast colour spray or blur with this trait, you can't cast that spell with it again until you finish a long rest. you can also cast either of those spells using any spell slots you have of the appropriate level.
Intelligence, Wisdom, or Charisma is your spellcasting ability for these spells when you cast them with this trait (choose when you select this race). None of these spells require material components when you cast them.

Quick lighting. When you hit a creature or object with an attack you can use your bonus action to target them with the Light spell. If a creature hits you with an attack you can use your reaction to target them with the Light spell. You ignore the range of light when casting it in this way, and if the target or the creature wearing the target is unwilling that creature must succeed on a dexterity saving throw to avoid the spell.

Light adapt. When you cast Light you can choose to have the light visible to everyone or only merfolk. In addition you can cast light on up to 3 targets at a time. If you cast it again when that would exceed your maximum the oldest cast immediately ends before casting the spell.

Detritus Eater. Aphotic merfolk have a long history of eating whatever meagre scraps they had access to, which has left them extremely hardy. You have Resistance to Poison damage. You also have advantage on saving throws you make to avoid or end the poisoned condition or a disease.

Weathering. You weather though times better then most. You consider your exhaustion level as one lower then it actually is. So the first level of exhaustion does not impose any negatives and you only die at the 7th level of exhaustion. In addition you have advantage on all saves to resist nonmagical sources of exhaustion.