That appears indeed, to be one of the many oligocheata, A group that includes bristle worms, earthworms, and other annelids. They are detritus eaters and since they are in your filter pads they are probably feeding on dead bacteria and partly broken down food and fish waste. I don't know of any freshwater ones that are harmful. They actually make pretty good snacks for molly sized fish. To get rid of themyou will need to rinse out your pads, and clean your filters a couple times until they stop reappearing. This does have the down side of damaging the benificial bacteria though. If you have a communnity tank of 2-3" fish like mollies, gouramis, etc., you could just set the pads in there one at a time and let the fish pick them off.