Looks like bryopsis which is very much a nuisance and one that requires specific removal or it just keeps coming back
Unlike most algae- you must get to the roots and remove all of them. Best tool is a dental pick or small crochet needle and pull them off
Best bud to do this in a container of tank water and finish removal by scrubbing areas after removal with a firm toothbrush and hydrogen peroxide
Return rock to tank and add cleaners such as:
Pitho crabs
Ninja star snails
Chiton snails