They put out a web like material to catch detrius etc. as it flows through the water. Their webs can sometimes bother corals that are close to them. They don't sting or anything but the webbing can attach to the corals etc. and that can cause them to close etc.. They can quickly grow and some have said when left alone they can take over the tank and become a nuisance. If they start to grow in numbers, you can break them off at the base, usually with a knife or sharp object. Be careful, the edges are sharp on the tube they make. I've used a female slotted spoon to break them off before. Do you have any pics so we can be sure we're talking about verm. snails?