hair algae is as mentioned a tough one to battle, at the end it's a couple things combined that end up eliminating. when it started developing on one of my rocks, I pretty much just ended up manually removing as much as possible, reducing my photo period, stopped feeding corals for 2-3 weeks, cut back on feding fish, and of course kept up on reasonably adequate water changes.
good luck, it's an ugly algae to have to deal with, but heres a thread from a couple months back originally posted by michika, try to get as much as possible out of it. and don't let the gha win!