While you have the chance, take it out. Stuff like that is good at disappearing into your tank if you let it. I agree that it looks like like a flatworm.
Kind of figured he didn't look friendly. I only noticed because my Royal Gamma attacked him. Went ahead and got him out. Thanks again for advice. I was hoping he was one of the Aptasia Eating Nudi's I had put in awhile back ago that I had assumed starved out, but he looked nothing like them.