This is an eastern fox squirrel (Sciurus niger). This individual is expressing a genetic trait called piebaldism. It is similar to albinism and amelanism where an animal's fur lacks pigmentation. However, piebald animals typically only have patches of fur/skin that lack pigment, giving them a strange mottled appearance.

Here are a few articles I found that discuss this rare occurrence in fox squirrels and other animals.

https://www.post-gazette.com/life/outdoors/2008/02/24/Five-questions-on-piebaldism-with-Jerry-Feaser-Pennsylvania-Game-Commission/stories/<personal data hidden>3

https://dfwurbanwildlife.com/2015/03/17/chris-jacksons-dfw-urban-wildlife/fox-squirrel-oddities/

Thanks for sharing this cool observation and hope this information helps!