Algae that is thriving can keep its surface clear of foreign growth. The specimen you have might not be in good shape. Do you have other algae in the system that is doing well?
I would try this, just reduce the time on the dip. Two to Three minutes. 75ml Hydrogen Peroxide to 2.5 Liters of the Tanks Saltwater for Six Minutes. Let Hydrogen Peroxide Solution, set for Two Hours, and return the water to Aquarium. If your Hydrogen Peroxide has been setting around for awhile, get new, as it looses it potency. Six Minutes is for Corals, to kill of Algae, Don’t be surprised if Hermit and other Crabs, start crawling all over the Macro.