Its really odd, I never had algae prior to this...
Some of the net research tells me that green algae is caused by excess light.... The brown is caused by a lack (totally simplified)
Found this link helpful: http://freshaquarium.about.com/od/algae/a/attackalgae.htm
(from that site)
What Causes Algae Overgrowth?
Like any plant life, algae thrive on three basic necessities: water, sunlight and nutrients. If an excess of any are available, algae will grow like wildfire, just like weeds growing in a garden.
Types of algae:
Already have algae in your aquarium, and need to get rid of it? Knowing the type of algae will help determine the cause and cure. Here are commonly seen algae types, and how to deal with them. For more information about a specific type, click on the link provided.
Brown, also known as Gravel or Silica algae. Common in new tanks, it will coat the tank in sheets, which are easily wiped off. It is usually harmless and will eventually go away as the tank matures.
Blue-Green, also known as Slime or Smear algae – Caused by excesses of nitrates and phosphates, this is actually not an algae at all. Instead it is a cyanobacteria. It can spread rapidly, and can cause considerable damage. Good water care will help, but if your water source has phosphates in it, you may have to use special treatments to remove the excess nutrients. Erythromycin is also effective against Blue-Green algae.
Red or Beard algae – This one is the toughest algae to get rid of, and usually appears on plants. A dip in weak bleach solution (5 to 10%) for a few minutes will often kill this algae.
Green, also known as Hair, Thread, or Spot algae – This is a normal healthy type of algae that every tank will most likely experience in some degree. As long as the tank is well cared for, it will not overgrow.
Green Water, also known as Algae Bloom – This is caused by growth of microscopic algae that are suspended in the water. It is one of the more frustrating types of algae to remove, as can't be wiped or scraped off like other algae. Generally water changes are not effective, as the remaining algae will quickly grow back. Use of a diatomic filter, or completely blocking all light for several days is usually necessary to conquer green water.