Thanks for writing. Have you cut holes in the sides and bottom of the garbage can for extra air flow and so excess water can escape? If not, I'd definitely do that to prevent problems down the road.
The mildew won't hurt anything --- but until you add soil, your kitchen scraps probably won't begin to compost. This is because most of the organisms that actually do the composting live in the soil --- so I'd definitely add soil as soon as you can. It's also really helpful to add some finished compost --- the finished compost will act like a booster to help things get started more quickly.
---Justin, Senior Garden Editor, BHG.com "