Most buildings with solid walls were built pre war when there was open fireplaces, drafty windows and were cooler with no central heating, this all helped to keep them dry.
The mold is caused by the damp that forms on the inside surface of a relatively cold wall, especially if unventilated.
A permanent cure would be to line the walls with insulated plasterboard - this can be bonded in place with dabs of adhesive,
however, you have to remove and replace skirtings, it would add a couple of inches to the wall thickness and you have to work around any windows.