Impossible is just a big word thrown around by small men who find it easier to live in the world they’ve been given then to explore the power they have to change it. Impossible is nothing.
It does sound unusual and I have also never seen this before on an aging dog. Hopefully, it is nothing and just something with his coloring that you don't have to worry about. However, the only way to be sure is to take him to the vet, just to get him a full checkup, to make sure that nothing is wrong. I hope everything turns out OK.
