Interesting. Perhaps he is a very neat biter? Tail-biting can vary very widely in appearance. Some biters prefer to only nip in between the rays, which tends to be the most common, some like to bite out large sections. Perhaps yours prefers a sort of even lawn mower motion?
Either way, if this happened overnight it is more likely to be biting--the dark and gray area, though it is hard do determine the texture of it, may be an area of infection. Is it crusty or fuzzy?
Either way, I would consider doing some additional water changes during the week, and adding a bit of aquarium salt.
Note: Epsom salt is not the same as aquarium salt, it is a diuretic. Alienbetta meant kosher salt, I believe.