Reef tank hair algae?

I have had an outbreak of hair algae in my tank. Im not sure why? I use RO water, phosphate free salt, no ammonia. so im puzzled? However i am still gettin phosphate makin this stuff grow from some where. any ideas how? also i added a poly filter into my wet dry filter today to remove the phosphates. how long can i expect to have to wait before the algae dies off?