We have had a continuing problem with mold on 2 of our outside walls. What could be causing the mold?
Mold grows when sufficient moisture is present. This could be from elevated humility in the room or it could be water vapor trapped in the walls. Another possibility is liquid water entering the walls from outside.
You don't mention the age of your residence or the type of wall system affected. Often older homes and buildings lack vapor barriers as part of the wall system and they often don't have much insulation. It is possible that some moisture is inside the affected walls causing the mold that you see. Moisture can become trapped when extra insulation is blown into a wall or leaks are occurring.
Investigate the condition of the walls outside and look for failed caulking and/or paint, especially around windows and doors. Visible rot could occur where water has been entering into a wall. Roof leaks is another point of water intrusion that can cause mold.
If everything looks OK outside another possibility is humidity in the room and lack of air movement. Is the mold growth behind furniture? Lack of air movement and humid air coming into contact with a cold wall can also cause mold.
These are a few possibilities. Try to find out where the moisture is coming from and fix any obvious leaks.
Well for now you can use white vinegar over the mold and get it cleaned. But for later on, I suggest that you firstly find the source of water or moisture and get it fixed. Usually the wall attached to the bathroom tends get mold formation due to water escaping into the wall.