How to long term store Sweet Potatoes
Cool dark and dry are your friends here. The mold is likely from the presence of too much moisture. A clean dry cotton towel on the bottom of the box or the bin will help control the moisture.
Any time you store something like this you need to remember that it isn't a "store it and forget it" set of circumstances. Look through them every few days. If there is a bad one, get rid of it. Clean any mold out of your container, as it is already contaminated.
New Crop/Freshly dug Sweet Potatoes need several weeks to properly cure in Kilns...
In a rush to get the crop to market often times the potatoes are not completely dried.
Wait a few more weeks before buying for long term storage...They will be drier, and sweeter too!!! At home store in a cool, dry, environment with some air flow...HTH