This one looks like a nudi.
Nudis are specialized eaters, meaning they can only eat one type of food (nothing else), and once that food is gone they quickly die. It could be a common bhergia nudi which eats aiptasia based on what I can tell on color from the picture.
It can also be one of the thousands of types of nudis that came as a hitchhiker. The hitchhikers usually die quickly as we don't have anything for them to eat.
at first glance, it looks as though it may be a zoa eating nudibranch. If you have zoanthids, it likey came on one of them. Even if you dipped, the eggs are not affected and will subsequently hatch within a week and infest your tank.
IF that's what it is, then where there's one, there will be many.
I have yet to do any treatment but my zoas r completely open? I saw my leopard wrasse picking stuff off them but I assumed it was algae. Could he have been eating these nudis all along?