I have experienced this one before around 3 years ago, I have a 20-gallon planted tank at my desk near the window and was not directly exposed to the sunlight. Days passed by, it produces algae but the colour is green and not brown but I know it is algae because the plants I added in my tank is just Java ferns, Anubias and I forgot the other names of the plant.
There are many types of algae. There are also a few organisms that look like algae but are not algae at all. In a healthy aquarium, the algae will be green, and if left alone, will grow hair-like in appearance. Algae that is red or brown may indicate a water quality problem, such as high dissolved nitrate or phosphate levels in the water. Red and brown algae are also common in new tanks when the aquarium is not yet ecologically balanced.
Sometimes, it looks really good to have algae but sometimes it's not because it grows and spread very fast. So what I did is I clean it up and transfer to another place that won't exposed by any light that came from the sun. I just used a 40watts light for my tank. Tried observing it for a day or 2 and a week after, I didn't see any algae in my tank anymore.
This is the picture before I clean my tank but I think I have deleted my pic after I clean my tank.