That coco look wet ... Real wet. This is going to be a over watering issue. J
Just watch her and let the pot dry out some. Seedlings this young don't need much water and it can be very easy to over water in a pot this size. Next time just water water around the seedling not the whole pot.
hey Hunnit, could be anything, too much light, too much water, too many nutrients, not enough cal mag. probably the last one if you didnt add any yet. Hope this helps ! 🚀