The wall is getting regularly soaked or water is condensing there.

Check the outside when it's raining to see if water runs across or splashes near that bit of the wall (doesn't have to be exact), and get a humidity sensor, if the room is humid, buy a ~£100 dehumidifier, not one of the tiny protable ones, a suitcase sized unit. It will take a few days running non stop to dry out the room, pulling tens of litres of water out.

The dehumidifier will make the heating more efficient, speed up drying clothes and stop mold as well.

Took about a fortnight to dry out our flat when we moved in and that was all from long term condensation.