It's probably malignant melatonisis. Prognosis is grim.
No, totally kidding. Dogs who are all white because they are SPOTTED white (and this includes white Shepherds, the whiter arctic breeds, etc.) still retain a lot of pigment cells in their skin. It's very normal as these dogs age for them to develop greyish spots on belly, tongue, around the mouth, etc.--anywhere the hair is very thin. They're all over her, actually, but it's just showing up on the skin that you can see through her hair. As long as the spots are greyish or peach, round like a dime or quarter (don't have to be perfect, but if they're all jaggy it's not a ticking mark), and plainly part of the skin and not raised or inflamed, I wouldn't worry a bit.
Merle markings are a totally different thing--merle is a fascinating gene that is a sort of incomplete dominant white-out gene. The merle gene destroys melanin, but it can't quite win over the normal color of the dog when there's only one copy of the gene. A Mm (one merle gene, one normal gene) dog has jagged patches of its real color (black, chocolate, brindle, whatever) on a background of that color mixed with white--so black becomes mousy or silver grey, chocolate becomes washy light brown, etc. The grey parts are where the merle gene has managed to kill some of the melanin. Normal merle dogs are happy, healthy, and fine--they just have beautiful and unusual markings. However, bad things happen when two copies of the merle gene meet. Now there is nothing stopping them from destroying melanin, and they usually make a dog almost completely white. However, unlike a healthy spotted-white dog, a double-merle dog has had the actual pigment cells killed or disabled. And pigment cells, in the developing embryo, also contribute to the brain, inner ear, and eye. So if there are not enough healthy pigment cells to go around, those structures are compromised. Double-merle white dogs are almost invariably deaf, many are blind because of microopthalmia (the eyeballs are literally too small for the sockets), and epilepsy is common as well.
In those breeds that have merle (Australian Shepherds, Collies, etc.) there are recommendations for breeding to avoid getting a double-merle, but some breeders still produce them. It CAN be worth it to get just the offspring you need, but you have to be pretty hard-hearted and willing to cull puppies rigidly if it turns out that they are blind; some also cull deaf puppies but others place them.
