Algae isn't always horrible. It looks ugly, yes, but it's okay to leave a little in there (there are also different types of algae, but other people know more about this than I do).
There are different reasons for algae, but the main ones are too much light and/or not enough water changes (so too much bio-gunk in the water). You could try just having the light on less. If the tank is near a window and getting natural light, you could try moving it.