It is damp around here and Splash spent the night standing beside some mares in another lot all night and when I cleaned his hooves out the next day they where pure white. You could scrape the white out with the hoofpick.. what exactly is this?
Can you post a pic of his sole? Could be that his sole is shedding naturally or overgrown; could be a fungus. Generally when I see that on a sole, it means that that part of the sole is ready to come off and I pare it out. Until you have an answer or a farrier out, keep his feet clean, brush them with a stiff brush and spray with 50% applecider vinegar.
I believe it's just the sole shedding out. I see it on our mares often, depending on how much riding I do, how much callous they have built up, and the changes in weather from wet to very dry. If you just want to be safe, putting the apple cider vinegar or iodine on the sole will normally take care of any potential fungus.