This happens from time to time, a ductless unit that provides such a good place for mold to grow: cold, dark, and moist, that the system must be cleaned. Don’t be upset with the system, it does not “create” mold! This means there is mold somewhere in your residence, become airborne, and found it’s way onto the coil/blower wheel. All of these product recommendations are purely from experience, and/or contractor success stories. Regular maintenance is important to halt mold growth in your investment. So, first let’s talk prevention.
Cleaning the air filters regularly will ensure proper airflow and stop the blower wheel from ramping up significantly, using more power, and pulling moisture off of the coil spreading the grounds the mold will grow.
Even better, during installation, consider treating the evaporator with a mold control product like BBJ Mold Control. Prevention of mold is much less expensive than cleaning or replacing equipment! Word of caution, most states require a license to apply this product. Take Massachusetts for instance: I was able to confirm application of BBJ Mold Control, an EPA registered antimicrobial pesticide, requires an application license (include passing said exam) from the MA Department of Agricultural Resources. Then take into account insurance premiums, etc.
Another option: consider immediately operating your heat pump in the heating cycle, once planning on shutting down your cooling for a long period of time. This will dry the coil and blower wheel and avoid creating optimal conditions for growth.
If things have reached the point of cleanup, mold is visible on the blower wheel and coil, the most effective way I have seen is use of the Mini Spilt Bib Kit from Speedclean. This kit will protect the electronics and wall surrounding the unit, giving the technician the opportunity to use a pressure washer like coil cleaner.
The alternative would be significant work disassembling the indoor unit, costing hours of labor to the homeowner.
Once cleaned, continued treatment to address the mold in the home is required. Otherwise, these conditions within a ductless unit in cooling will continue to recur. If this was my home, I would start with finding high humidity, dark areas like the basement and any crawl spaces.
Installation of a whole-home dehumidifier is a necessity in any New England basement. I have found it close to impossible to keep up with the smaller, reservoir type in my own home.
Once controlling humidity, killing any mold that is not visible could be performed by installing a Global Plasma Solutions’ Cold Plasma Air Purification Device. These small IAQ wonders can be located within your ductless indoor unit, and wire directly into the unit’s power, taking just minutes to install (in most cases).