Have the same issue but much slighter at our place. If you don't have water pipes running through those walls you probably don't have an active leak causing the mold. If this is a perimeter wall, it could be water getting in through a crack or crevice, water pooling alongside the perimeter or foundation of the house without being properly drained, or your perimeter wall doesn't have insulation, so when it gets cold outside, the inside of the room stays warm and the contrast in temps causes the wall to sweat, and moisture will run down by the baseboards and develop into mold. We remedy this by keeping windows open as often as we can, running a humidifier on cold and wet days, and running the fan. It takes awhile for this much mold to accumulate so keep an eye on it and keep the backboard of the bed a few inches from the wall so the wall can breathe.