First time I've planted supertunia bordeaux in garden soil within in a planter. I initially watered with some miracle grow. After a couple weeks of watering there appears to be developing a "yellow dust" on top of the garden soil. When I water the plants the yellow matter appears to "steam". The plants seem fine, but not sure what the yellow dust like matter is or if I should be concerned. The picture of the yellow matter is what it appears like after watering. It certainly seems odd.

Its probably a fungus. When you water, the spores drift.

After searching for different terms, I think I figured out what the yellow dust is. I believe it is the beginning of "dog vomit slime mold". Yuck. I'm off to the store. I knew I should have purchased top soil, but I liked the look of the garden soil better. Any confirmation or additional thoughts? I'd love to know more!

Most garden soils are supposed to be mixed in with your soil as an amendment not used on their own, they retain to much moisture. You should use potting soil in a container for correct drainage.

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.