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Looks like bubble algae - carefully pick it off with tweezers. Do it outside the tank if you can remove that rock, be sure not to burst it in the tank.
You're welcome! I had some bubble algae on the base of a frag I made sure to get that off as soon as I saw it lol. It looks really soft and squishy but it was pretty hard, like a small glass ball. But be careful - if you do burst the bubble it will spread alllll over your tank...