Update! Rascal went in to the vet for a teeth cleaning, and after looking at his nails the vet decided it was some sort of undetermined fungal/bacterial infection. So he prescribed some sort of antiseptic soak for Rascal's footsies, 5 minutes, 2 times a day, for.... 21 days . Anyway, he agreed it's...

I was cleaning off Rascal's feet a bit and noticed he keeps getting these weird marks on his nails and this weird dark-colored stuff at the base of the nail. The stuff at the base of the nail could be like old skin cells or something? It scrapes off easily with a fingernail. But the marks on the...