Mold build-up is everywhere. There were instances mini-splits system gets mold, and it will be a continuous problem. There are kits to protect the wall and space while cleaning them. You can try to get it clean; this mold growth often depends on use. Lift the units away from the wall and spray behind them with mold killer. Another suggestion would be spraying a full quart of 70% Isopropyl Alcohol into the Coil. You can do this after cleaning. Some Do it Yourself Procedures Make it a habit to clean the unit every once in a while. Take the group out every couple of months, and wash it for about an hour. Clean as much dust and dirt out as you can, and don't forget to take off the panels. Disinfect and spray into the inner workings as best you can. Do not forget to clean the air pipe behind the control panel. Please shut the unit down, unplug, spray more into the inner part. Plug it in, and crank it back up. Make sure to run on high speed for testing in a couple of minutes....