[Turbo snails are also a good, natural way to help get rid of the algae.
I have this problem, too - though maybe not to the extent that you have - and it's due to the high phosphate level of my source water. (Even after the RO/DI)
I also use the PhosGuard pellets in a nylon bag, and that helps, too. You can't leave it there on a continuos basis if you have coral, so I do a one week on, one week off.]