I was going to also ask if you are sure that it's hair algae and not bryopsis. Take a closer look at it. Bryopsis looks like small ferns. If it is hair algae, try hydrogen peroxide. Look up the correct way to dose it. Trying to treat the source after the outbreak will be hard since the nutrients are locked up in the algae. Try the peroxide. When the algae dies back, big water change with RODI water.