When plastic plates/bowls sit for a few days they get this black coating on it. What is that?
It doesn't look like dust to me
It's soot. Plastics have more of an electrical charge than many other surfaces, so they attract more of the soot in the air, and if they're white plates, it's especially visible. To minimize exposure to soot, keep the plasticware stacked in a closed plastic bag, and pull the ones you need out from the center of the stack.
That's all well and good for the plates, of course, but the owner of the plates, who presumably has a pair of lungs, might start to wonder why there's so much soot in the air. That might lead to a non-random question, which I suppose would need a different thread.