Musing:
Possibilities that spring to mind are moisture (water), moisture (other), or extremely dry (static electric attraction).
I discount static because it doesn't look like dust that's held together by static, it looks like dust that's held together by moisture.
If it was static, I think it would look much looser, softer, fluffier, instead of having that "smeared by putty knife" look to it.
My money's on brine that got grated out of the rock by the grinder. Reminds me of a carrot juicer. (I remember the first time I juiced a carrot, I was amazed that so much liquid could come out of a "dry" carrot.)