It appears to have green oxidation meaning it is either copper or brass. I don't recall electrical boxes being made from brass or copper so I would rule that out. I'm thinking either an early copper coin or large flat button. Tough to see in the one pic if that is a scar in the center where the shank would have been attached.
Once you get the rough crust off, rubbing a little 3 in one oil on it and holding it at an angle to the light might bring out some detail if there is any. Problem is a lot of early coppers were very pocket worn before being lost so you have an uphill battle getting an ID on them sometimes.