help! what is this

[czwicky]

This is the third one of these things ive seen in the last 3 days. They look almost like a nudibranch or a snail without a shell. I'm sorry for the bad pictures he wouldn't move out of the curve pf my tank. Could someone please tell me what these are and if they are harmless or really bad?!

 

[Bonessurf]

Looks kinda like a nudi... I'd toss it.

 

[czwicky]

That's kinda what I was Thinking but idn where it would've come from