Throughout the winter I keep my shop around 55*. If we get a warm day in the spring and I leave any door open for any length of time, condensation builds up on the car (moist warm air - cold object). Its less visible on the interior but obviously its there and this is what breeds mold and mildew and from one small spore it breeds like wildfire.

Why one of your cars and not the other? Was one completely closed (windows, etc.) and the other left open? Maybe one gets more heat than the other or is further from open doors. Who knows! Cars that get more use also tend to have less of an issue - like daily drivers !!

There are some pucks on the market that you can put in the car while its being stored but they probably "prevent" mold moreso than kill it since it did nothing for mine (and it was as bad as yours!). Although I have to admit I don't know what "dead" mold should look like !! Once you open the package you're not supposed to breath the gas it emits so you have to leave the car sealed up until you want to use it Which I suppose is a good thing as it will keep most of the moisture out.