Possibly Aiptasia. These things are the devil.
There are many ways to take care of (kill) them, Joes Juice, Aiptasia X, Kalk past, lemon juice, boiling water...all of which you inject into them with a syringe.
Whatever you do act now before they invade your tank. They sting corals and reproduce sexually and asexually. What’s worked well for me is to put a dab of coral glue on my finger (which is just gel super glue) and smear it over the area where the Aiptasia is. The issue with this is if it’s done large scale, you have a ton of glue all over your rocks which isn’t aesthetically pleasing.