Damp/Mould

My tenant has reported a patch of mould next to the window in the apartment bedroom where they sleep; it first occurred about 3 weeks ago and is about 3cm2. They are wiping it off each morning.

The apartment is typically very warm without the heating on and they started putting the radiators on in November. I have asked if this room is ventilated and whether it is cooler than the other rooms. They do not have the radiator on in this room as they like to sleep in a cooler room and do not open the window due to the extreme cold outside at the moment. I believe both will be a factor in the problem and have suggested they ventilate when they are not asleep; I would appreciate your advice on how to deal with this matter as mould/damp can affect health.

Many thanks