Agree with above, it is clove polyps. They are a nightmare as they cover any available surface in a reef and will crowd out other corals. Best to address them now before they become a bigger problem. I would suggest removing the rock from that plug and putting on a new plug. Then cover the remaining rock surface with super glue gel like stated above, without getting it on the rasta zoa. Worst case fenbendazole will kill the clove polyps, but depending on what else you have in your tank, I would suggest doing that treatment in a QT tank.