The stain in the wood under the lacquer (clearcoat, whatever) has faded - you can't get to that stain unless you remove the clearcoat lacquer. No amount of buffing will help unless you strip down to the wood, practically. A refinish is what you'd need to do to remove it completely, and that isn't cost effective, IMHO.
The sticker is not 100% my style, but I'd rather just leave it, or leave the shadow. Or maybe remove it, put the guitar out into the sun for a while and see if you can fade the "shadow" while not impacting the rest of the finish! (Not likely, though.)