My Mystic has the same thing except her's are brown and sort of crusty (sorry). I asked my vet if it was anything to worry about and he said she was just fine. He said that some cats have very wet noses and others are drier....Mystic just happens to be a drier nose. He said it was like a humans "booger" dried mucus that catches the dust.

Sorry, I had to be a little disgusting in order to explain it.