Spagetti worms live in the sand. Web worms and those 2 tentacled worms live in the rocks most of the time. The hard calcerous web worms, Vemetid snails, will harm your sps and aggrivate your other things with their "fishing rod" phlem strands. I break them off with needle nose pliers and throw them out for fertilizer. I agree with JimD, undesireables for sure.