At a minimum, I would take a 5 gallon bucket filled with tank water and a good stiff brush or toothbrush and scrub the rocks in the bucket. Obviously, you need to be careful with the corals. I did that on some rocks and it worked pretty well, and mostly eliminated the nuisance algaes. Make sure you have a second bucket to rinse the rocks in before you put them back in to the tank as well. It makes a real mess.