Nudibranch
Another rule in the sea: anything brightly colored should be avoided. From my reading these creatures store poison in vesicles, which it releases when threatened. So the bright markings are a warning. The shot you need is to see the horns and the tuft. These two guys were not too far from one another. I lifted a prayer rug and there they were. Prayer rug? Yes somebody discarded a rug into the sea. And the one on the white coral is striking in contrast. The actual size is about that of your little finger nail.
photobackstory 