I have lots of experience with hair algae, unforunately, but I have conquered the problem I had.
First off, my water parameters always came back excellent. PH, calcium, alk, phosphate, nitrate, nitrite, salinity, temp, etc, etc.
I blame the beginning of my problem on some LR that I got that had some on it. I had snails, hermits, starfish etc.
Here is what I did/changed.
I took off the MH's and put my PC's back on.
I bought urchins, a lawnmower blenny and lots more snails, especially the big turbo grazers.
I did two 50% water changes in a 4 month period.
I took out ALL of the LR, and there was about 200 lbs of it. I scrubbed off the pieces I thought I could save with hot water and a little toothbrush-shaped hard bristle brush. The rock that was truly covered got tossed. I had to do the rock cleaning thing several times.
I put a second skimmer on my tank. I added phosphate sponges to the filters. I cut down the time the lights were on to about 6 hours per day for awhile. I cleaned, cleaned, cleaned the sides, rocks, eqiupment every time the hair algae would start re-appearing.
I lost several very nice coral pieces from all the moving around and cleaning I did.
But after almost 7 months of dedicated work, I can say I'm back to my beautiful tank I had before. Anytime I see one little piece of hair algae popping it's ugly little head, that rock comes out and gets scrubbed.
Good luck, it's a battle, but it can be won.