i got an old credit card and scraped it of (very tiring, but don't give up until you clean it all of) and if there's some on the rocks just leave it there cuase it looks cool, but don't worry the next step is gonna SLOWLY get the algae off the rocks.
STEP TWO: dose the correct amount of flourish excel which is used to add co2 to your tank to SLOWLY get the algae problem cleared up.
but if the algae makes the water murky you can just let it sink to the bottom but i'm fussy about the clearness of my water so i stuck my spare filter in and the two filters working together worked a charm, but if you don't have a spare filter you could try a vaccum...
cheers, Eric.