Sorry, I can't see anything through the plastic. The pic would be better if the animal was in a lid of tank water. It's probably an algae and detritus eating Nudibranch.
Eating something on the sand, means it's probably not harmful. In fact 99.9% of the animal species we find in our tanks are beneficial, not harmful. I always leave things until I see them causing harm. That means I rarely remove animals from my tanks.
A LFS employee told a hobbyist that they had to buy a Harlequin Shrimp to eat the Asterina Stars. The LFS didn't listen to the hobbyist say that the Stars were eating algae and bacteria on the glass and didn't even stop to ask the hobbyist if the Stars were bothering anything. Be careful who you listen to.
Aloha
Edited by Mark Peterson - September 22 2014 at 4:09pm