This is a common question we get. Unless you have wood windows, mold does not grow on vinyl or fiberglass windows. Mold is growing on the dust and fed by moisture. Condensation (sweating) of the windows due to the extreme difference of temperatures (-10°C outside versus +18°C inside), paired with the lack of ventilation and humidity inside is usually the cause of the mold growth on the windows.
To prevent moisture and mold from developing on the windows, you must ensure that:
Since this is a relatively minor issue, you can remove the mold by yourself using a product called Concrobium Mold Control. It is available at many big box chains such as Home Depot and Canadian Tire. Make sure you follow the manufacturer instructions (read the label). Concrobium will leave a residue that does not allow mold to grow and that is the main reason we recommend it versus many other products.
If you have wood windows you must sand the affected areas to completely remove it.