Annoying little worms. Makes the rock sharp enough to cut you if you handle it bare handed. They use those little filaments you see to grab fish food. They are normal with live rock and don't build a tube that sticks up longer than about 1/2 an inch. These little things don't really hurt anything, but make live rock very sharp if the fish try to scratch off something thats bothering them. If there is a cluster of them in a spot up current from an sps and they cast filaments into the water, it can annoy the sps coral a lot in that area.