It just got too much moisture in it and the mold is a sign of fungal decomposition of the organic matter in the soil mix. Any potting soil is liable to generate them as generally all potting soils will have some sort of OM included in the mix. The fungal growth is not harmful to the plants.

Just an FYI, but there is a difference between planting soil and potting soil. If your planters have a bottom to them, you want to use potting soil rather than planting soil. Miracle Gro soils are not great products for any number of reasons. But they are not alone in developing fungal growth or mushrooms - any soil has that potential. And fungus gnats should not be a concern with outside containers.