For color film to retain maximum fidelity over time, it has to be archived and stored properly. Humidity and temperature, as well as industrial pollutants in the air are the main culprits. If you are really serious about your archival storage, and you live in relatively damp climate, you might consider the purchase of a room dehumidifier for the room you intend to store your film materials in. Here, where I live we do have relatively humid, hot summers, but with central referigerated air conditioning in my studio, the air stays relatively dry, and I have never experienced the mold issue you have. The basement of my residence, being partially below ground level, requires the constant running of a dehumidifier in warm weather, or I have mold growth on things.
Also, if I am not mistaken, Kodachrome slides have a factory applied lacquer, which other types of slides do not. It is this lacquer that attracts mold growth, if I remember correctly.