if the worm is super fast, it could be a bobbit worm, that is a nasty worm.. i had 1 last time, over a foot long, and it ate about 5 fishes before i found out it's there..
what happened was, it refused to come out, so i poured boiling water over the rock, and splitted the rock, to my amazement, a cooked 1 foot fat and long nasty sea centipede.. a way to id it is that it has fangs in front and feelers.. and ten times faster than a normal bristle..
just my experiences, hope that helps..