Look like aiptasia. Ugh! You want to get that rock out and maybe try to scrape off the aiptasia and then poor h2o2 on it and then set fire to it! Next, poor glue all over it to seal every crack, dip in muriatic acid, and then maybe think about drying it out for a year, bleaching and then put it back into the tank.
heehee
I am exaggerating, obviously. But, you do not want them to multiply in such a small tank. At what you have now, get a true peppermint shrimp to eat them up. Or, pull rock and scrub off and douse with h2o2 without hitting coral.
KP Aquatics is where I get my peppermint shrimp and they have always gobbled up my aiptasia. Fast.