If you have a light on it, which you probably do... try turning it on as little as possible... the fish don't need it to be happy, they will know day from night from any other light in the room.
Also get something to scrub the majority of the algae off, there are various types of brushes that work great-- Just make sure you get a glass one for a glass tank, or one made for plastic tanks if the tank is plastic, otherwise you could make it less pretty.
Consider also changing the filter pad, as algae also tends to grow on it, and it will just help seed more algae.