While it's a good idea for your farrier or even veterinarian to take a look, if it is thrush, as I suspect, my thoroughly worn copy of Natural Horse Care has this to say:
"The back half of the frog peels off in deep layers, and the frog is narrow rather than being a wide, healthy triangle. The diseased tissue is light gray to white in color, or black between the peeling layers; healthy frog is medium gray".
Treatment can be very effective... one that's recommended and that we've used on cow ponies successfully here in the western U.S.is:
"After trimming away all "shedding" or peeling frog material with a sharp hoof knife -- sharp so that you can cleanly cut away the diseased part. You may see deep, black cracks; trim till all the black is gone and all remaining frog is open to the air. This will let the treatment get to all infected areas. Your horse may be sore in the heels for a few days, but frog grows back really fast when the fungus finally gets treated.
Followed by using Borax cleaning powder (a laundry product, such as "20 Mule Team Borax" or "Boraxo"), which is available at hardware stores and builders supply stores or at the drug store (ya'll call it "chemist", no?) Borax works by changing the pH (acidity) of the frog to a more alkaline range so that the fungus is unable to reproduce. Borax is found to be successful, easy to deal with, and inexpensive.
Use 1 Tablespoon borax powder in a bucket with enough warm water to cover the foot. Add 1 dropper of Calendula tincture, an herb that helps with skin conditions, and soak 15 minutes, 4 to 10 times over several weeks to a month. On the in-between days you can soak a cotton ball with this solution and stuff it into the central sulcus. You can stop soaking when there is obvious new growth in the center of the frog."