FLATWORM!!!
They are nasty, get rid of it asap, and all others you find. They spread like crazy, and are toxic. Nothing really eats them. What ever you do do not use chemicals to kill them. WOrker in my LFS used chemicals, his dog got sick, besides everything in the tank died. He had so many of them, the water turned brown from the toxin, and his dog was ill. They are nasty, only thing that eats them is a velvet nudi branch, and good luck finding one. I suggest you hunt for them asap.