it definitely sounds like it although I would describe the dust more grey in colour.The mites do not live on the birds but live in cracks and crevises and feed of the birds at night.There are various sprays and powders you can buy but unless you iradicate them from the loft they will reappear.Poultry shield is good you dilute it and spray into all their hiding places,it seems to dissolve them and you will see a green stain where this happens.If the birds are then treated with the drops or one spot type treatments you should be able to keep on top of it.