A healthy growth of green algae in an established aquarium, a sign of good water quality, will happen naturally. Microscopic algae that appear as greenish "films" on glass are easily cleaned with a rag, or controlled by herbivorous species. Snails are often thrown into a tank to thwart any overgrowth, which usually works well. Overgrowth of filamentous algae, which look "hairy," can be harmful and should be removed by hand as much as possible to prevent take-over. Look out for these troublemakers.
For more information, please read the story How to Keep Algae in Check.