What finally killed off the hair algae in my display tank was a multi-pronged approach.
- Dipping rocks in peroxide helped, but wasn't permanent
- Setting up a refugium with Chaeto helped
- Running GFO helped
What finally got rid of HA was removing half of the live rock, dipping in Peroxide, then putting it into dark storage bin with saltwater, a pump, and a heater for several weeks (called "cooking rocks"). I swapped the rocks between the tank and the storage bin after a few weeks, so they'd all been cooked. It hasn't come back. I think when I first set up the tank, I had a major nutrient problem, and all the GFO and Chaeto wasn't go to remove the phosphate fast enough to kill the HA. As I understand it, cooking the rock burns up the phosphate.