Can you take a picture? It depends horses have what's called Periople it's an off white rubber stuff on the outside of the hoof wall, normally at the top, you can scrape it off with a hoof pick, normally a farrier will clean that up when he does it.
But if you notice a ring or a distinct 'bump' growing out of the horse's hoof those indicate nutritional changes. If you're worried, which pictures will help us determine, but I think your farrier would honestly know better than most of us - unless someone on here is a vet or farrier? Which there may be. But if you're worried you could put him on a hoof supplement, they don't hurt. I have all my horses on Brewer's yeast which provides a great deal of all the essential B vitamins - it helps keep hair and hoof properly nourished.