first ... the picture is not clear enough for me to tell anything, so I am directing you here (based on the color in your image) ... is it this? viewtopic.php?f=2&t=118890
Looks like a bristle worm to me. When you pick it up with tweezers does it shrink up and get really spiny? Can you get a pic of it against a solid white back ground?
That is a bristle worm. ... most are beneficial, but I prefer to have none as I really like having baby snails every now and then. When I had bristle worms in my old tank -- never had this little wonder pop up every now and then.