Allershausen is correct - the mould is not caused by anything that you are doing wrong. There is probably moisture leaking in from outside somehow.
Mould in a "dry room" can be caused when the outside wall of a house is left to go too cold. When warm moist air from other parts of the house hit the wall it condenses creating the perfect conditions for mould. You need to heat the effected room properly over a long period of time so the outside wall gets back up to temperature and can dry out the moisture. Airing the room during the process is essential to get rid of the moisture. Once you've done that (and gotten rid of the mould), you can have the room cooler than other parts of the house, if you want to, but it is essential that it doesn't get tooo cold and that the door remains closed to prevent the draught of warm moist air. Airing the room is essential but don't keep the window open permanently, just open it for half an hour once or twice a day; enough to refresh the air without letting the walls of the room cool down too much.
Also it is important that air can circulate around the room so you shouldn't have large pieces of furniture pushed directly up against an outside wall. Always leave a few centimetres space to let air circulate.
This happened to me in a previous apartment.
I used to leave the bedroom window open most of the time as i hated sleeping in a room where i couldn't breath and liked to keep the air fresh. So it didn't get too cold, I left the bedroom door open. The lounge was heated and air from this room flowed into the bedroom where it hit the cold outside wall and condensed and caused mould. On the advice of an expert who came and looked at it, I fixed the problem by cleaning the area with bleach then drying out the room using the process described above. No more mould afterwards.