That's just the crud that has encrusted onto it over the years. We usually dug copper and brass with that in old hardwoods. Lot's of times if I dig a coin or button with that I just wait until it dries and then flick it off with my finger nail and it's clean. If there is no sign of a shank on the back I think it is just a well worn copper. I think it's a coin hold it at different angles in the light and see if you can see the outline of a head or something like that. We dig a lot of early coppers and large cents that are really worn and there is no way to tell exactly what they were.
No offense to anyone but I never put copper coins in lemon juice. I've seen them lose detail using just plain water. I would just flake the brown crust off and if it all comes off leave it like it is. If it doesn't all come off peroxide should be safe to use on it.