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Mold is everywhere. Keep the candy in an air tight container. Mold spores travel through the air.
There are some ways to reduce mold in your home.
Clean your refrigerator every few months with a food safe cleaning solution. One recommendation is to use one tablespoon of baking soda in three quarts of water. Rinse with clear water and dry.
Scrub the rubber seals of your refrigerator with a solution of three teaspoons of bleach in a quart of water. If the seals look black, there is likely mold there.
Keep dish cloths, towels, sponges, and mops as clean as you can. If they smell even a little musty they can be a prime environment for mold to grow.
Keep the humidity in your house as low as you can. This can be difficult if you live in an area that typically has high relative humidity.
As far as an additive for preventing mold, I have little or no experience in this. We keep very little candy on hand, homemade or otherwise.