I agree it's probably some sort of invert larvae, but hermit crab larvae I've seen have been fairly energetic, move in sort of pulsed movements, and very commonly bring their tail under their front (sort of curled into a ball), so I wouldn't immediately expect those. They don't have the spike on the front, so probably not porcelain crab, and they don't paddle around more calmly like a kind of lysmata shrimp. They also don't appear to have smashing appendages or anything, so not mantis shrimp. Maybe a different type than the blue leg/red leg hermits I keep is possible, the shape and energy level is about right.
I would not guess fish of any type, they don't look like clowns, and your other fish are either pelagic spawners or spawners that lay their eggs on a surface to have them fertilized - since you've only got one of each, even if the eggs were laid, they wouldn't able to be fertilized afterwards (and they likely wouldn't hatch in such a coordinated burst as an invert shooting out babies).
Did you start with live rock? What inverts have you added? Do you have a pistol shrimp?
