When I first set up my tank, I had this BIG, BEAUTIFUL piece of bleached coral. Then it got covered in diatoms, then green algea, then some other crap. Anyway my point is that is, that it got ugly and when it did, I put the piece on my back patio... in the sun. And lo & behold, 2 days later it was white again!! But by then I had already made up my mind to go with live rock only. So sunlight will clean your algea problems, with no ill effects to the tank.