That 'cotton' algae is a real pest. You might try a small urchin. If you can 'free' a bad rock from the structure, and take it to your sink, immerse it side to side to side (30 sec each) in hydrogen peroxide just as it comes from the drugstore, then rinse vigorously in tap water, you will have an impaired rock, but an algae free one, and if your rock is old enough, it will repopulate the surface with bacteria from inside or from the sandbed. Be VERY VERY careful to rinse well and start the rock in your sump for about an hour, so it will shed whatever pure oxy bubbles it retains. Pure oxy is like acid. What we breathe is hugely diluted. So you do not want a bubble to get into fishy gills. Repeating this treatment when a rock gets out of hand should ultimately get it. But BE CAREFUL. Oxy in pure state is lethal to all living things!