Yep, that's the reason you're seeing slime mold, Matt. Garden soil is meant for plants in the ground, not in containers.
Potting soil is a whole different animal, and as Bob says, it drains much better, has lots of organic fiberous materials in it that expand when wet and contract when dry so there are air spaces in the soil, and they don't compact.
Hate to say it, but I think you should start again using Potting Mix. Carefully remove the plants, dump the container and rinse it well, then fill with new potting mix and re-plant your plants. You might want to rinse some of the soil off the plants, too, just to try and remove any remaining spores of the fungus.
