i don't think that mold is a verb.
A real answer Edit
The spores that grow mold are very small (most too small to see easily), swirl everywhere and are commonly found in normal air. They often land on bread where they use the bread as food to grow. There are various kinds and can cause bread to taste bad.
Many commercial breads have a preservative which kills mold and generally won't harm people in the small concentrations it appears. It will prevent mold for quite some time. Home-made breads will mold faster because they usually lack preservatives (which otherwise makes them good). You can prevent bread mold somewhat by storing your bread in a cool, dry place. Bread mold should not be eaten, as some types can be quite toxic.