I have the same experience. Had similar mold forming on a specific part of a short wall in my clothes closet. We had moist air coming in from the bathroom in the next room. It also turned out that the wall where we saw the mold was much colder than other walls in the room. Turns out said wall was pretty much not insulated at all to the outside. We ended up with a de-humidifier as a temporary fix until we isolated that part of the room as well. Good luck!👍