+1 to vermetid snail. Not necessarily harmful by themselves but can multiply very very quickly covering all your rocks. They will irritate corals as they feed with their webs. To get rid of them there are few options. You can cover their opening with any cyanoacrylic glue, dollar store glue works fine. You can break them off at the base and destroy it, not as easy as it sounds, some strong forceps can usually do the trick, but you have to get the entire thing or it will grow back. Or I have heard mixed opinions on bumble bee snails that will eat them.