From FrugalReef

The definition of the word algae is surprisingly complex. It's an old word that's gone through a lot of revisionist thinking. A simplified definition of algae would be to say they are marine plants. But not everything you might consider a plant is considered a plant. Coral, for example, is considered an animal, even though most of the species we think of as coral are both photosynthetic and non-mobile. But as far as marine aquarium keeping goes, that definition will have to suffice. Check out wikipedia if you want to learn more about how organisms are classified as algae.

Algae Quick Identification Guide

Light red brown dusting on the glass and rocks are usually Diatoms, especially in aquariums less than 6 months old.

Light green film on the glass and rocks is Green Algae.

Dark purple, red, green or black slime on the glass and rocks is Cyanobacteria (Aka. Red Slime Algae).

Dark green hair on corals and rocks is called Hair Algae.

Light pink, purple or orange spots are called Coralline Algae.

Green bubbles fixed to rocks are called Bubble Algae.

Algae Control

The primary method of controlling algae is to limit phosphate. Phosphate is required for algae growth. Tap water is a primary source of phosphates. So is overfeeding. So the first steps in algae control are using only RO or distilled water and not to overfeed. (Note, if the RO Filtration system isn't maintained properly phosphate elimination will suffer, if you suspect this purchase a phosphate test to confirm. RO water should not contain phosphate.) If you cannot eliminate phosphates in your incomming water use a commercial phosphate remover to treat it.

The next most popular method of algae control is to add specimens to your tank that will eat it. This will keep it from building up on the rocks and glass. Snails and Hermit Crabs are an excellent choice for this.

If you need to take algae control one step further, you can permanently remove the phosphates and nitrates it needs by growing a macroalgae like Chaetomorpha in your sump, and trimming it when it gets too large. This also has the advantage of absorbing ammonia and nitrate.

Long term, encouraging the growth of Coraline Algae will help significantly, because it releases chemicals that retard algae growth.