He might have something internal. Monitor him and make sure that he has food and water. It could have been a fight or a fall. The mud isn’t that unusual for a cat but it could be an indicator if he’s not wanting to clean it off. If he’s still acting off after a bit I’d take him to a vet. His leg could be broken or something. If he doesn’t clean himself that could also be an indicator. Bless his heart. ♥️