Gray or speckled snow mold, proper name Typhula Blight. (Or mould. Canadians can't decide how to spell that one.) It's a mycelium that appears when the snow's been on the ground too long. It's showing on a lot of lawns right now, as the snow disappears.It's sort of pretty, in a weird, Spanish-moss sort of way. It doesn't kill the grass, just damages the blades (and then hides for the summer).
Are you seeing this where you live?
(Photo found here)