Hair algae normally takes over when there are access nutirents in the water. Have you tested phosphates and nitrates? Are you using RO/DI water that has been tested with a TDS meter? The best method I have found to get rid of hair algae is to remove its food source. There is also another possibility that this bryopsis which will live with better water quality. Do you have a picture of this algae?
Can you not buy ro water at your grocery store? If you are using tap water you will have problems. Most stores sell ro water in one gallon jugs. Try that and when you feed make sure food never hits the bottom. Fish are pigs and they will try to eat everything give them.