It's Bryosis
From being old school, I'd treat it with Brightwell's Hydrat-MG magnesium.
There’s many different kinds of nuisance algae that can cause problems in the reef aquarium and Bryopsis can be one of the worst, or one of the ‘best’ depending on your perspective.
reefbuilders.com
But a lot of reefer's have been using fluconazole.