Careful application of a diluted bleach and water mixture. You could mix a couple of caps full of bleach into a gallon of water and use a sponge to apply it to the wall. As long as you are careful while applying it (avoiding letting it hit the plants), your problem should start to disappear. Try another application a few days after that.

As mentioned, you need to find out why your wall is molding. Is there any known source the moisture could be coming from?