It's most likely yeast which is harmless. It's the same stuff you see on grapes when you buy them. The mites are related to the white powder in a manner of speaking since they are there to eat the yeast.
If this coating can be rubbed off it sounds like powdery mildew.Powdery mildew is actually caused by high humidity .It looks bad but rarely will kill plants.
Treat mites by using an approved mite remover by Orto products. You could though have a type of mold which can be easily removed by washing the plants with mild soap and water. Make sure you treat each leaf.
Well, yeah I think it is caused by mites for sure. That is the clear indication. Anything white on the plants has to do with mites or any other fungus disease.