Have you tried to wipe that off with a plastic cleaner or Goo Gone. It looks as if there could be slight movement in the batteries causing the plastic mating surfaces to Fret or rub against each other. Or it could be just the action of sliding the battery in or out. Make sure you are sliding the batteries in evenly and not on an angle. Careful examination of the cleaned battery surfaces with a magnifying glass will show you if there is any physical damage to the batteries.