Nothing more joyful than mold build up due to insufficient airflow in a space that has a lot of humidity generated.Remember, rent is not a gift to you.

I conduct water and energy efficiency analyses as well as airflow testing to ensure adequate ventilation in the project buildings.

(Why you won't open the bedroom doors to let some air flow through is beyond me).Today, it's late paying tenants and a repair person that won't call me back.

@Atul KaushalGet some airflow thru there somehow.

At the very least there will be no airflow through the furnace and it will shut down on its high temperature safety controls.

The insulation can help as well, and also making sure the soffit vents are clean and not obstructed so that the attic gets sufficient natural airflow so it does not get too hot.  

Hence, my concern that finding this mold airborne it is very unusual.Possible SourcesIt is possible that the moisture is from outside air leaking in ... and if there is a gap/opening in a perimeter wall or window, then that could create the airflow that would cause this mold to become airborne.  

The wall is cold and without warm air flow, moisture collects on the wall.

Here is some speculation , if the house did not have central air , and it was added to an older existing heating system ductwork , that duct work is insufficent for the airflow of a central air system .  

Hvac tech hereChange the filter, call it a day.As others said, dont expect anything from someone cutting you a check to liveIf its a furnace, the heat exchanger can crack from poor airflow (extreme cases)If its a heatpump, the indoor coil can get clogged if filter was installed improperly, or got sucked through due to high resistance.