Hair algae is always a pest I think it is a requirement at the 3 month mark its like a Reef gods test of your commitment to the hobby. I passed the test and this is how I did it.
1. Granular Ferric Oxcide GFO
but I used it in a DIY reactor
2. Manual removal of the algea as much as you can at a time while doing a water change. Shut off all of your pumps and pull off tha algae and suck it out with a siphon.
water is a must. Dont skimp on it get a unit for your self especially if you have a larger tank it will pay for it self in no time just in gas for trips to the LFS
4. Dont overfeed, if you think you arent overfeeding you probably still are overfeeding.
5 test your water in the morning before the lights come on the day after you clean out as much algea as you can. This will give you a better reading on phosphate levels as the GHA hasnt had time to consume what is in the water column
6 be persistent
7 Natural algae eaters are helpful but you must fix the base issue, seahares and urchins are both great at removal.
8 try to stay away from chemical herbicides
9 set up a fuge and run a macro, any algae that you can grow to outcompete the GHA for nutrients is a great way to starve out the GHA.