Gray "Dust" On Pink Insulation in Attic
I've been in no less than 300 attics and every one of then has had dust, grey dust, in them. Your attic has soffits installed to vent the attic. You also have vent pipes going through the roof to vent caps that allow some backdraft into the attic. If you've ever had your roof redone, stuff will be knocked down. Mold needs a fare amount of moisture and most of the attics I've been in just don't have it. 

If this is going to keep you up at night, you can always have a piece of the insulation tested.