The exhaust fan is directly above a stacked washer dryer. We don't turn it on very ofter. I notice that this dry, brittle, pressed powder like substance collects on top of the dryer. I pulled off the fan cover and the blades are covered with it as well. When i poked around it all started to fall. Anyone ever see this before? What is it? I am going to have the fan removed and the duct checked. It vents straight back to the outside (not attic.) Thanks.