There are only two possible reasons for mould appearing on the inside of walls of accommodation. One is that the adopted lifestyle of the occupants is inappropriate/unsuitable (inadequate heating, too high atmospheric moisture with inadequate ventilation - the latter commonly being overplayed to the point of demanding draughts, piling moisture in by drying washing on radiators, cooking without extraction, closing individual rooms into sealed cells, etc. are common features). The other is that the accommodation is genuinely unfit for habitation, often simply because of its lack, more likely practical absence, of thermal insulation of walls (although that would not be openly stated in the UK where, sadly, such housing abounds).