This is acid corrosion ,you need to neutralise the acid by washing with a alkaline solution like mentioned baking soda or something with similar properties. smear Vaseline petroleum jelly over the terminals and over the exposed metal,petroleum jelly has alkaline property's and I believe it would do a better job than grease in this case,don't put on the post and terminal contact points it may effect conductivity .Terminals and battery post need to be cleaned properly for best contact.