It's not powdery mildew if this stuff is growing on the soil. PM only grows on plant tissues. A white substance on the surface of the potting medium could be algae, fungus, or soluble salts.

First leave the covers off so the surface of the soil can dry out a bit. If it is warm enough outside put you trays outside in direct sunlight, the sunlight and breeze will help kill the fungus. If your plants are starting to keel over from damping off fungus try sprinkling ground cinnamon on them. If any place around you sells Pro-Mix with biofungicide use that for starting your plants. It has some kind of friendly bacteria in it that eats fungus. But get fresh stuff, the bacteria in one year old stuff will be dead.

Californian:
Thanks for your reply. This was the first time I experienced the fuzzy fungus on my see tray soils in 10 years. When I first discovered it I took the covers and gave it sun when the weather permitted. This took care of the problem.
This year I used MG seed starter mix instead of Jiffey. I will go back to Jiffey next year and if I continue to have this problem, I will try to find Pro-Mix with biofungicide.
Thank you for taking the time to answer with constructive comments.
Bob