Sea spider. It will eat not just sps but also lps and zoas too
they usually come out after lights are down and hide among the rocks in the day. You need to treat your tank with interceptor tablets. They showed up out of nowhere in my tank a few months ago and they multiplied fast! I treated it with one interceptor tablet for large dog each time for 3 times and that took care of it. I have a total of roughly 230 gal tank and sump.