Answer

By Your Horse

Farrier Glyn Trundle advises…..

Frogs vary dramatically between horses. Frog growth generally correlates to hoof growth, and a horse who grows a lot of hoof wall tends to grow a lot of frog.
The frog grows in layers, and sometimes it just peels away or falls off to leave a good, clean, healthy frog underneath like you describe. This is very common and nothing
to worry about.
If however, the frog bleeds or looks sore or uncomfortable, you should consult your vet or farrier.
The effect of horses ‘shredding’ their frog can be exaggerated by boggy, wet, or muddy patches in the field.  I don’t think the rocky patches are significant in this case but if the rocks are quite exposed and sharp, then it would be worth (if possible), fencing off that area to avoid other foot injuries such as bruised soles.