Galvanic corrosion is where you have two dissimilar metals in contact with resulting corrosion at the contact surface. Galvanic usually eats away at the aluminium pretty badly (you get pitting in the surface). Sounds like you have some oxidation ocurring on the surface which is not as bad and is usally limited to the outer surface only. You can give it a light polish with a scotch brite pad and that should remove the oxidation, however they usually leave scratch marks. It would be better to track down a metal polish at your local hardware or auto shop that will remove the oxide and leave a better finish. Try and dry it out first before putting it away as standing water will lead to the same problem again.