Pretty sure neo was right, there's a whole family of them do some more research on the Nemertea I bet you come across the same one. One thing I did read was that thy are not round they are flat sounds a lot like what u mentioned .
Lol sorry for the new addidition kyle..in case anyones wondering i gave kyle all my fishh and coral with a bunch of rocks so im guessing it came from me ..maybe its the thing i happened to see a few times that i tried to id with no luck lol problems yours now