Have you tried a different test kit cause hair algae means that your phosphates or nitrates are high...
Well there are a couple of things you can do...
1. Get a tooth brush and keep lightly scrubbing the area while trying to syphon as much of the algae that that is breaking off... Keep this up and after a while it will keep reducing and disappear.
2. Use some kind of an object to block the light from hitting that area if the rock... The algae needs light to survive... It will starve itself to death.
3. Increase the water flow to the area... Algae mainly forms in dead spots in the tank where dirt mat settle and start to decompose...
4. Clean out your filters regularly... If you have live rock that matches your water volume (1 pound per gallon) and if you have ceramic beeds or bio balls then take them out as they are not needed and they can cause lots of algae problems in the tank.
5. Install a tv sterilizer this kills harmful algae causing bacteria.
6. You can also try introducing an emerald crab to the tank.
Hope this helps and good luck!