Every saltwater aquarist experiences Green Hair Algae (GHA) in their marine aquarium at one time or another. It may start as just a few strands of algae on a piece of live rock and seem to not be a big problem, but if the conditions in the aquarium are right, it can spread across that one piece of rock, then leap to other pieces and then quickly jump to the aquarium walls and cover them.
Most people will try to scrape it off the glass with algae cleaning magnets, brushes or even razor blades, and the live rock with brushes, but that only slows it down and can actually make it spread even faster as the bits of algae will come to rest all over the tank.
Taking the rock s out of the aquarium and scrubbing it off the rocks seems to help for a bit, but it will usually come back in a short period of time unless the rock is literally sterilized with an oxidant like bleach. Even then, if all of the algae is not removed from the tank, it will start all over again. To say the least, it can be a very frustrating experience.
What You Can Do About It
The good news is that there are a number of things that you can do to your tank which will slow the algae growth down. Reducing the foods that the green hair algae requires to grow will stunt its growth. Most algae feed on nitrates (NO3) and phosphates (PO4) so removing these from the aquarium water can make a difference in its growth rate. It is virtually impossible to remove all nitrates and phosphates from your tank water so don't add to your frustration by shooting for zero levels of these. Everything that you put into your tank, including fish and critter foods will raise both the nitrate and phosphate in the tank water.
Both Natural Sea Water (NSW) and the Sea Salt mix that you buy to make the saltwater you put in your tank contain levels of both (PO4) and (NO3). Decomposing food and detritus generate these compounds, so keeping your substrate clean is a good beginning to reducing the algae food.
How do you keep your substrate clean in a saltwater aquarium?
You can siphon clean it, but that can be a pain as well as difficult to do in a reef tank (all that live rock gets in the way). Fortunately, nature has provided critters that will do the job for you. Sand sifting Gobies are fantastic sand scrubbers. They gobble up the substrate, suck whatever non-sand items are in it, then spit it out through their gills. Not only efficient, but fun to watch as well. The downside of these cleaners is that they require a smaller grain sand (not crushed coral) in order to pass the sand through their gills. The Sand Sifting Sea Star does a great job of gobbling up "stuff" from the substrate, however, it will also consume all of the live material in Live Sand at the same time. There are a number of Reef Safe Hermit Crabs that do a great job of cleaning the substrate, also.
You can perform endless water changes to reduce your phosphate and nitrate levels, but many have found that the Vodka Method For Nitrate (and Phosphate) Reduction works well if you have a protein skimmer in your system and there are a number of Nitrate Reducing Products which also reduce these compounds.
Marine Critters That Might Help
There are a number of marine critters which consume Green Hair Algae.
There are quite a number of different species of algae which look very much like the typical Green Hair Algae found in saltwater aquariums which are not the same. The differences can only be identified via a microscopic examination by someone who knows what they are looking for. These different types of algae, even though they look like GHA are distinctly different to the normal algae eating fish and will not be consumed. If you are experiencing this type of GHA and no eradication method seems to work, your only course of action to get rid of it may be to strip the tank and scrub everything down.
Green Hair Algae also requires light to grow, but greatly reducing the light in a reef tank can have a detrimental effect on the corals in your tank.
Some reduction will help reduce the growth, but cutting back on your lighting will not significantly reduce your GHA rate of growth.