Your breathing at night produces a lot of moisture. Its enough to make the windows moldy. There is probably not any difference with this window and the others. Do you have an ensuite? Do you thrown damp towels on the rads in the main bedroom.
All these will add to the mositure.
Good ventilation is the key. A dehumidifier is useful (it needs to be a proper electric one, not a gel refillable one), it helps to remove the water without having to improve the ventilation, but it needs to be left on a lot and they do use electricity.
Try opening the windows a bit and see how that works. Leaving the ensuite fan on for longer is also very improtant.