That is a large polychaete worm. They are colloquially called "bristle worms" but that is like calling a specific species of rabbit a "bunny."
Some people don't mind them, but personally I think they are heinous. I have taken great lengths to get them out of my tank, not only because of how they look but their bristles are extremely fine and will sting/burn the skin on contact.
Like the other guys said, it will be essentially harmless in your tank's ecosystem.