Green Hair Algae, or GHA as many forums refer to it, is a bonafide nuisance in any marine aquaria. The bad news is, it is always an uphill battle against it. Good news is, unlike cyanobacteria, there are many more creatures who will happily gobble it up.
|Green Hair Algae|
In case you did not read my voice on cyanobacteria, with the exception that brown slim algae is not algae, just about everything else on the characteristic of cyanobacteria is the same as green hair algae- will thrive with light and dissolved nutrient. The one key difference is that this is an actual algae and will require iron, among other elements to properly grow. So on top of light control, nutrient control, and predator introduction, we can add one more element here: competition.
Unlike cyanobacteria that can disappear completely if you keep light off for an entire day or two, hair algae is an actual plant and can store enough energy to sustain an extended period of darkness, just like your corals would. So the light off trick must be carefully modified here- instead of all out for a day or two, you can attempt to change spectrum/color temperature and also shorten photoperiod.
Reduction of dissolve nutrients will not affect corals so this is a safe approach by limiting how much you feed your fish, inverts, and corals, and maximizing your skimmer.
Predators- Many creatures naturally prey on hair algae- all tangs, most dwarf angelfish (reef caution), all rabbitfish (reef caution), many hermit crabs, snails, urchins, etc.
Lastly- competition. So a hair algae’s competition is obviously another algae- but you probably don’t want more algae in the tank? a good option is to house some macro algae, here is a place you can find many different varieties with reasonable price: Reef2go- Macroalgae. Keep these macro algae in your established sump or refugium to absorb nutrient and minerals just like hair algae would, but at a much faster rate to out compete the hair algae.