Looks to me like a bristle worm. Does it look... 'fuzzy'?
If so, do not touch it with your bare hands. They are generally harmless, but that 'furr' is fiberglass-like bristles that are hell to get out of your fingers. Use tweezers. If you do get them on you, duct tape.
On a happier note, they are amazing bottom-of-the-foodchain decomposers. I like to have a lot of bristleworms in my tank because it incredibly helps any bioload issues on my filtration (in a natural way), but they creep out a lot of my friends.