I don't know of any fungus infections in chickens' feet. Fungal infections are more common on combs and wattles or internally. I really wouldn't do anything for this. I have old fashioned Rhode Island reds who have this coloring. Most hatchery chickens have different breeds in their backgrounds, so I really wouldn't be concerned. Since it is on both feet pretty equally I would say it is natural coloring. It probably isn't desired coloring for showing, but he is still good looking.