I'm new to this hobby, so I can't say for 100% sure, but it might be a bristle worm? I had a similar looking worm Hitchhike into my tank. A lot of people leave them alone, b/c the consider them to be part of the clean up crew. (They eat detritus). I don't think they are an issue, unless they start reproducing into large numbers. I've had one in my tank for several months, and not noticed anymore. I'm sure someone else will chime in, but you might google bristle worms, and see what you think. I decided it was best to leave mine alone.