Mold is a product of excess moisture, not temperature, so humidity and dew point, are the issue. Moisture can come from outside air( humidity), capillarity ( slab wicking ), dew point( think pulling a glass item from the refrigerator) , leaks ect ect. Mold spores are everywhere ( space station smell bad from spores, condensation, and skin cells). The A/C blows about 25% RH tops, heat close to the same.

My thoughts are that I would leave the windows open for ventilation. I would be concerned about leaving the ceiling fan on because the motor could overheat and cause a fire. It makes no sense to spend $4,000-$5,000 for a mini split unit in a small room that you don't use. Just my opinion.