Hair algae thrives even in healthy plant conditions if I recall. I think healthy plants will outcompete them eventually but if you don't have enough, they may still be there. Algaefix works really well on it but just watch the dosing and dose gradually over a few hours. Fish and shrimp do not like it. If you notice stress, water change immediately.
So first, add more plants, if that dont work, you can try a blackout but I think algaefix is probably the more surefire way or eradicating it. I hate advising the use of chemicals but i had a hard time with hair algae as well and my plants just were suffering too much from the algae so I needed soemthing else. Algaefix did it for me.