it likely is condesnation on a cold spot but just check the brickwork on the outside at the location, repair any cracked render or repoint any brickwork with dodgy pointing. A coat of something like Thompsons water shield to help the rain water run off may also help.
Then as above, control the damp produced inside, good extractor fans in kitchen and bathroom or using windows, don’t dry clothes inside if possible, and move air about the house by opening windows, make sure any airbricks are not blocked, even install one in this spot/close to it.
If you want to see the effect of any of changes or actions without waiting to see if mold will reappear after cleaning, get a cheap clock and weather station with humidity, they are only about a fiver. They will show how the humidity goes up if you shower with the window shut etc, and help you identify what to change.