Join Date: Mar 2013
Location: Montgomery Village (back to NYC soon!)
The algae is utilizing the light along with excess nutrients in the water to grow; due to the small amount of plant bioload in your aquarium, the algae is going to continue to grow unless more plants are introduced, or water changes are done regularly to reduce algae promoting nutrients. I would vote the first of the two choices.
You can also try to start dosing Flourish Excel according to directions to help reduce algae growth, but know that even this is only a band-aid if you have continually high levels of nutrients in the water combined with high lighting and a low plant bio-load level.
Hope that helps