For lack of better pictures it appears it could be a creature of the order: Nudibranchia. If that's what it is they tend to be prey specific, (feeding on certain items only), be it corals, sponges, algae, other nudibranches, flatworms, etc. Without the proper food items they usually starve to death in our reef tanks.
We do need better close up pictures to even begin to narrow it down.
