Mould coming through the floor.?

We have been in our house for a few months. It is about 80 years old with original wooden floorboards. The air comes up through the cracks in the boards, but in one of he rooms I have noticed that mould is creeping onto anything that is left in a particular corner. I left a pair of shoes there for 3 days and they started getting the green dusty mould on them. The area is really cold to touch even in the height of summer. It is in a corner of the house very close to the ground. There is a side vent to the outside and the house stands on brick foundations. What is it and how can we fix it?