ok time to post....my guinea pig has that on her paws. I never got it checked because it looks like a result from her having her paws in dirt all the time. No matter how clean a piggy cage is there's still poo pellets she steps on.
I don't want to suggest your dog's dirty and needs more cleaning. I don't know what he's around, whether he isn't washed after enough , or whether this "thing" is a result of dirty fur. I have no idea
If vet doesn't know...who would??but they don't know everything. maybe it's something that can be removed surgically?