Vermetid snail, try to take it off from the coral skeleton, but if, do it from the base! If you are scared about to take it off gently, then put over it/ dab it with super glue, if gel better, if liquid small amount over not to leak over the skeleton
It will start to irritate the coral with that slimy strings (which it use to feed catching particles from the water current) also it can spread it self fast enough to stress most of your corals.