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.
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.