that stain is rust oxide,likely there was a piece of iron near it...a nail or such?I would leave it,but thats me...the coin is fairly worn though so the Value has been degraded,so......if you just have to clean the stain off...take a small amount of powdered Cream of Tar Tar Powder and slightly moisten it,then lightly rubbing the rust will take it off.Be careful as visible scratches can be left. Great find,I love seein big silver in the HOLE!!!!