agree with ausiefish
could play with the photcycle a little if you are sure the chemical levels are OK...one thing you can do to kill it off (i had to do this with hair algae in my nano) is turn the lights way down to around 2 or 3 hours a day (i actually went black for 2 days, but i didnt have any delicate photsynthetic corals, just some shrooms and polyps) to kill the stuff, changing out maybe 10% of the water each evening and after those few days do a 25% change and sratr running a phosphate sponge (I used Kent Marine's product) in a HOB filter for a couple days...the thing is, even if you read no phosphates if you have extensive algae growth it wont show until you kill the algae and re-release the phosphates, same prinicple for nitrates/trite...
also, as ausie said may sure your topoff water and new saltwater is not full of nitrites/trates or phosphates
i would ignore dizbjon as he has no idea what he is talking about
KB