How to prevent mold in stored joint compound
Mold grows on any surface that provides food for growth. Joint compound contains organic compounds and lots of moisture. The compound will only last a finite length of time even under "ideal" conditions (if mold spores don't ruin it drying-out will).    To slow down its' expiration time: when storing the compound for any length of time, place the plastic that was included with the compound so it completely covers the top layer. Next, soak a rag in a mixture of 1 gallon water to 1 tablespoon of bleach. Wring it out and place it on top of the plastic. If the lid for the bucket doesn't have a rubber seal place a plastic trash bag on the rim of the bucket and seal tightly with a mallet.