Hair algae, and most all unwanted algae, for that matter, comes from unwanted nutrients in the system. Phosphates and nitrates are normally the culprit. If you test for these andd see what they're at, there's a chance that they're at zero because the hair algae uses them to grow. You can help yourself by getting a good cleanup crew consisting of mexican turbo snails and assorted hermit crabs. You can also use a sterlized, brand new toothbrush to scrape some away. When or if you do this, be sure you have a mechanical filter that sucks the free algae into a trap that prevents it from floating around and reattaching and regrowing. You can also help yourself by making sure that your top off and change water is RO/DI water and NOT tap water. Tap water is notorious for having nutrients that hair algae wants. You can also make use of a refugium that grows caulerpa and or chaeto. As a last and continuing measure, do weekly or bi-weekly water changes.
Hope that helps.
