If what you are seeing in your saltwater aquarium is a green, feathery looking mass, it is probably green hair algae. Once green hair algae gets a foothold in a saltwater aquarium, it can soon cover everything in your tank if prompt measures are not taken. The cure for green hair algae is the same as the prevention: starve it into oblivion. Green hair algae require not only light but also nitrates and phosphates in order to survive.
Nitrates can be introduced into an aquarium not only as the end product of the nitrogen cycling process (ammonia/nitrite/nitrate) but also via some brands of commercial sea salts and tap water. Phosphate (PO4) is a part of life on earth. Virtually every living thing contains some phosphates and they can enter the tank in a number of ways. Fish & critter foods, tap water, and carbon are some of the phosphate generators in your tank.
If you have green hair algae in your tank, your phosphates should read well above .05 mg/L., which is considered by many to be the lowest level at which green hair algae will grow. If you have green hair algae in your tank and your phosphate and nitrate tests read low or near zero, the algae in your tank could be eating and locking up the phosphates and nitrates so they won't show up in a test.
How to Reduce Phosphates
The most common source of phosphates in an aquarium is from the freshwater used for top offs and water changes. In this case, doing water changes to reduce phosphates will only continue the problem unless the water source is changed. There are a few methods of reducing phosphates in your tank:
Use only RO or RO/DI water whether you purchase an RO/DI unit or purchase RO/DI water from a commercial source.
Purchase and use a good nitrate sponge. Many of the nitrate absorbing materials also absorb phosphates.
Use mangrove plants in your system to reduce phosphates.
Consider using the vodka method for reducing both nitrates and phosphates in your tank.
How to Reduce Nitrates
Nitrates will always be something to contend with in your tank. For fast, immediate reduction, you can do a water change, using the instant nitrate reduction method. This will reduce your nitrates at the fastest rate, using the least amount of time and water. This will also get your nitrates down to a workable (10 mg/L area) level but it is only a quick fix and does nothing to remove the source of the nitrates. Siphon out as much of the green hair algae as possible. You will probably find that you will have to use several other methods, many of which are part of a good aquarium maintenance routine to keep your nitrates in check.