Moldy Ceiling are unsightly for visitors but they are happening every where in air condition space if they are not taken care. But they do take time to form and start spreading.
The above moldy ceiling maybe due to condensation form around the supply air duct. It may also be due to water dripping from above the supply air grill due to poor insulation. This may not be resolve timely due to the cost involve to do the repair and made good the ceiling. But this will cause the indoor air quality to drop due to the presence of mold.