[Peterbug]

Algae only grows where there is food and moisture and time to grow. The north side gets less sun, so stays moist longer. Dust provides a food bed, even if it is very thin. After a little dust an moisture, the first layer of algae forms a thin biofilm designed by millions of years of evolution to assist algae in holding territory and gathering more dust food. Then growth really happens. After a good cleaning, a wax may reduce the moisture retention a little, as well as reduce the grip of the dust. But there is no magic to understanding the need to regularly manage the three basic ingredients of food moisture and time.