hair algae help

There are many ways to skin a cat. The key is solving the root issue. Normally GHA (green hair algae) feasts on Nitrate and Phosphate, also Ammonia and Nitrite which usually aren't present in a cycled tank. Those may also leech out of rock and be consumed immediately by the algae. Sometimes you can test 0 for N and P and still have algae due to this, and also because the algae is removing it from the water column as it is added by other sources (fish waste, food, etc).

Controlling the N and P is the key, whether that be controlling it via feeding techniques and/or filtration. You can black out the tank and kill off the algae, but really all this does it dump the nutrients it has absorbed back into the water where it then needs to be removed. You can manually remove it but this is sometimes not possible due to coral growth on the rocks or generally just being a total pain, and it doesn't address the root cause. You can get a clean-up-crew that can take care of it, which works well, yet it doesn't solve the root of the issue either, which is excess nutrients. But extra buddies in your tank eating algae are also cool to look at. Carbon or Purigen helps remove waste also, and GFO will knock the P down quick. Skimmers take out some organics and bacteria, essentially removing the excess food before it becomes waste (which is why some recommend shutting the skimmer down during feeding)

Also check your lighting, old lamps can shift to red which is what algae likes.

I'm an algae scrubber advocate personally, because it uses the enemy as your friend basically. You can set up a small scrubber for supplementary filtration quite easily and give the GHA a preferential place to grow. Algae removes inorganics, which is the food after is has been consumed (and this is what you test for). If you have 0 N and P and hair algae in the tank, then you have an algae scrubber already, it's just in your tank where you don't want it. You can combine it with various forms of filtration, most all work together except biopellets or GFO.

Like I said...there are always plenty of ways to attack it.