It is difficult to tell just what you have but if you have put any live rock, coral rock, etc from the Caribbean, then, my best guess is a sea hare possibly Aplysia Dactylomeia (spotted sea hare, Caribbean sea hare). if that is what you have they can get up to 5". they need lots of tender greens, will not eat caulerpa. (you might try some sea veggies, or seaweed select after it has soaked and become soft, put a little at the head and see if it takes it. good luck.