Mineral salts (Calcium Oxylate) that leach out of the tobacco over time. Tends to run in veins throughout the pressed tobacco. It's just one of the things that occurs as the tobacco ages, nothing to worry about.
It does seem to occur sooner/more frequently in Hoggarth/Samuel Gawith blends; I guess maybe I'd attribute that to the nature of the African tobacco used in a lot of their blends.
I wouldn't attribute it to any added topping/casing. I don't see heavily cased American aromatics growing crystals profusely...
These plugs are several years old. No appreciable topping/casing. The look like biscotti I'd get with my coffee at Caribou...