first time Houseowner - Need some help (moulding?)

[xD3S1x]

below picture is from one of the rooms. Over the winter when heating was turned on this is what happened... (i think it was heating over the winter but not sure)

Spoiler

My master bedroom nothing has happened yet in box room and 2nd room (mould?) happened on window side of the wall and on the windows...

what is the reason for this and how can i fix it?

 

[xD3S1x]

Do you dry your clothes in the house over the radiators during the winter months?

yes but not in the rooms it happened.

@Nightglow how can I improve the Ventilation? In winter windows have to stay close to keep the house warm... sorry am new to this.