I'm far from an expert on mold but I'm sure that is is related to moisture. When moisture (water vapor) is in the air in a gaseous state it wants to condense, it condenses on cold surfaces. I'm sure the mold grows there as a result of water vapor. If you can remove the water vapor either through heat or air movement I would guess your mold problem will disappear. If its a painted or porous surface the mold or bacteria that it grows from may be growing as a result of trapped water vapor on that surface. Perhaps if its a closed room like a bathroom you need a bigger exhaust fan. Or you can nail a new 5/8" drywall ceiling on top of the old ceiling and increase the density of your ceiling. The heat transfer rate would slow down and the moisture probably wont concentrate there anymore.