What on earth is this thing?????

[MrGoodbytes]

If it's a polyclad flatworm, chances are it's going to eat something you like. Best to play it safe with these guys and evict.

Then again, I had a big one in a 10 gallon back when I started but didn't see it really after I first noticed the surface of my rock moving. I guess whatever it ate that I would have liked, I didn't have yet :P.