The discolouration is oxidation of the copper caused by damp air or soil and this turns the copper into Malachite. This basically means that the coin has a thin layer of malachite where the copepr has oxidised.
This being the case the coin cannot be brought back to its pre lost condition and will never have any collector value except as a curiousity but in its own right has character and is a great wee treasure to have found.
Leave it as it is except a clean up in soap and water - NO abrasives, polishes or other nasties like sand paper! Coins can be cleaned in olive oil but int he case of a coin with a malachite oxidation it wont make toomuch difference.