Moldy food can at the least cause a horse to colic and at the most kill the horse. The reaction will be based on the type of mold and how much the horse ingested. You should never feed moldy food to a horse.
Frozen food is food that has been cooked and frozen the regular way. Snap frozen food usually applies to vegetables and is food that has been frozen very quickly. Freeze-dried food can be anything that has had moisture removed and is completely dry.
If you don't throw away food, the food will rot and become moldy. This will not only create a very undesirable smell in your household, but will also attract maggots, flies, and possibly small rodents.