Buy animals that like hair algae as a first line of defense. Sea hares are amazing, but if your water chemistry is bad they will wedge into a hole, die, rot, and ink.
Another method I've been tampering with is using epoxy. The places that almost always get covered first are areas by cracks or mini rock structures that a toothbrush and snails can't reach into. So, like using spackle to repair drywall, I crammed the epoxy into the area smoothed it out until it was a nice flat surface that little algae could not hide it. Any algae that grows on top of the epoxy was then readily accessible to the late Om-Nom (my old Blenny). I usually apply a coral as well so it will grow over the area.
Other than that, I wish I still had hair algae problems. I am living with bryopsis algae. But, the extreme methods for bryopsis also work on hair algae so I'll elaborate on what I've done and observed.
To curb it long enough for my plan* I first cleaned the rocks as best I could with a toothbrush, then made 1 part 3% peroxide plus 3 parts saltwater (0.75% Peroxide) and soaked the whole rock with coral (acros, mushrooms, montis, zoas) in the solution for ~5 minutes, rinse it thoroughly with saltwater to try to wash away debris, then put it back. I've never killed a coral this way, and it massacred the algae. If the rock was large enough I just basted peroxide over the surface and avoided allowing it to soak into all the holes and cracks, otherwise you will find many little creatures squirming out of the rock and that rock will become a big phosphate/nitrate bomb. For big rocks and corals I've also shut down all water flow and very slowly and directly covered the area with cold 3% peroxide mixed with a little salt** for a minute, then turned on the water flow again. The algae turned white and disappeared from the area in 2 days. Montis did not tolerate peroxide treatment as well as other species; I had a monti lose a little color for a few days but it recovered. I've noticed that generally more slimy corals are more resistant to the effects of peroxide. I also always make the corals retract and close up before treatment to minimize exposure to the more sensitive areas.
Next is vodka dosing to be sure to starve the algae, especially since there will likely be a large amount of dead biomass that you don't want to simply re-enter into the cycle. Be careful though, eradicating all the algae will starve some of your clean up crew, and if they start dying they will become large sources of biomass too.
*So far in my plan I have filled the hard to reach areas with epoxy (as mentioned above) and put some encrusting species over some areas to smother it. To take care of a couple large cliff edges I have I put in plating montipora to basically create an umbrella to shade the lower parts of the rock. I've also installed a refugium and filled it with macro algae.
**I use cold peroxide mixed with salt so the peroxide solution, which is less dense than saltwater, doesn't float upwards away from the target area.
This is my order of escalation. If you have any questions please ask because I'm really tired right now and am not 100% confident that I've been clear about all this...
