I have 2 HVAC units, one for upstairs and one for downstairs. When I went to change my air filter for the downstairs unit I noticed some mold in the return duct. This is a large duct that travels probably 25 feet up into the attic. Why would there be mold at the return? You can't see it on the outside. It is only visible if you remove the filter. The humidity at the thermostat consistently reads between 45% and 55%. My understanding is that this is a normal range.
Mold can be a problem with ac duct work especially when the condensate is not properly drained. I have found many systems that were properly designed but never serviced so water sits in the tray catching dust, then stuff starts growing plugs the drain line further and can make a real mess. I would make sure your system is draining properly if it is draining I would make sure to clean an sanitize it including the evaporator coils. I would be looking for worst case if everything is dry that the evaporator maybe icing up (this happens with an incorrect charge) due to low Freon and the air is not dehumidified compared to normal operation. The last and most extreme thing I have done is add a UV lamp to the duct to kill the the buggers (mold and mildew) many commercial buildings have UV sterilization systems. These are the things I would check. To kill the mold that is there I would not use my favorite mold killer 3%hydrogen peroxide because acids are not good for the coils. I use a foaming non acid coil cleaner like nu-brite, a pain in the butt to use on indoor coils but it will kill the mold clean the coils and sanitize the tray and drain after cleaning I flush with clear water