This is a normal reaction with the atmosphere.
You can use baking soda and a wire brush to clean the terminals and then after reassembly, smear some type of grease over the connections to seal them from the air. Your auto parts store also has little tubes of Dielectric Grease for this purpose.
CAUTION, be careful how you administer the baking soda. If ANY gets inside the battery cells you can kiss the battery goodbye.
Next, when removing battery cables for cleaning, Always remove the Negative Cable first and then the Positive to avoid short to ground. Reassemble in the reverse by installing the Positive first, then the Negative.
Dennis & Carol
2003, 4006, 41' DutchStar,
Spartan, ISC 8.3L W/BanksPower - 2013 Honda CRV, BlueOx Baseplate, Aventa Bar & Patriot Brake. And the 04 Bird, Sunshine Car.