Show us the edge, please, if any detail is left. And give us the weight. Is it attracted to a magnet? These are all standard questions for any coin that purports to be an 8 reales.
Looks like a well-worn piece, is all. I don't see any indication of it being counterfeit. These Spanish reales circulated in the New World for over a century. They're often worn so thin that I've heard people in the metal detecting community refer to them as "razor blades". I've dug some pretty slick ones myself, on old antebellum sites. Some worse than this one.