Buttermilk pancake mix is green?
Mold doesn't grow that fast in a refrigerator overnight. I'd bet some ingredient reacted with your baking powder or baking soda. I've seen something similar before in three separate circumstances:


instant oatmeal that turned blue when mixed with iodine-treated water (day after day on a camping trip);
garlic mixed with yogurt turns blue when heated;
blueberry pancake batter turned green overnight.