Most Elysia sp. are quite small and the ones I've seen hitch hike in on sponges are green to gold and eat only algae.
What color is it? (Hard to tell from your purple pictures.) Color and close observation will help ID what it eats as
well as the species. What did it come on? If it came with zoo rocks or a gorgonian, don't let it in your tank if you
have zoos or gorgonians and watch for hatchlings.