Have you noticed a black powder on your supply grills. This could very well be mold. The cause of the mold is the grill is getting too cold. When the surface of the grill has a temperature below dew point (around 58 degrees) condensation will form. This moisture allows dust particles to collect on the surface. The dust particles are growing medium for mold. Over time the colony expands to a visible black powder. What makes the grill get below dew point? Usually it is poor air flow. Poor air flow can be caused by a dirty or too restrictive air filter. The blower fan may be dirty and not running up to speed. The cooling coil may be restrictive and blocked with debris. If you notice a black powder on your grills, the underlying problem must be address. Just cleaning the grills will only be temporary as the mold will return. Have your system checked by a professional air conditioning technician. He will research the cause of poor airflow and correct it. Then have your duct system cleaned by a professional duct cleaning company.
Preventing mold is easier than eliminating it.