Diffuse as much light as possible. Does the sunlight hit directly onto the tank? If it does, you should move the tank...whether you want to or not is a different issue. Moving a tank whatever the distance is can be an all day affair...lol. Direct sunlight can change the temps of the water. The light though does not need to be direct to create almost uncontrollable algae. Algae eaters can only keep up with so much. Also, different algae eaters sometimes focus more on certain types of algae that may or may not be provided naturally at all times.
Phos-X by Hagen is a good, safe product to help with algae control. It's a filter bag of medium that absorbs nitrite, nitrate, and phosphates. The light control is up to you. You could tint the window or cover the area the light hits the tank with a piece of background.
How big is the tank?