how to i stop mould on internal walls?

Mould has started growing on the inside of exterior walls. Its in all three bedrooms upstairs. It is growing from the skirting boards up in the corners of the room and from the roof down again in the corners. In one of the bedrroms it comes at the bottomw of the wall but not in the corners. I cleaned it off about 2 weeks ago and it has come back again. in 2 of the rooms the mould is like a dry powder but in one of the rooms it is more damp. we have lived in this house for 8 years and it has only started appearing the last 9 months i keep cleaning it but it keeps coming back.