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:

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

This is normal. The batter is not bad and is a chemical reaction. Just stir it up. I have been eating pancakes for years and always refrigerate the extra batter. Its good for a couple of days.

Was it just a layer of green on top or was the mixture itself a green colour? The former would suggest it went mouldy; the latter that some chemical reaction occurred to alter the colour. You can also give it a good sniff... if the smell is at all unpleasant or noticeably different, this can be a sign that something went off.

Even if it didn't go "bad", I'd probably still toss it and just make half the amount next time, for a few reasons:

1. The change in colour might also indicate a change in the flavour.
2. If the batter is green, your pancakes will likely come out an unappetizing colour too.
3. I never enjoy eating something that I'm wondering if it might make me sick. It's just not worth it.
4. The pancakes will almost certainly not rise well after the batter has sat overnight (anything with baking powder should be cooked as quickly as possible after mixing the baking powder with the wet ingredients).

this happens every time I make buttermilk pancakes, the next day the batter looks grayish!! I mix it up well and cook with it and it's fine! I had some tonight for my dinner!! I always use Martha Stewart's recipe.

It's funny, because my pancakes never rise well on day one of the batter mix. Then after standing overnight.. I notice this chemical reaction... I used to think it was mold... but others are right. That is too fast to become mold. But, the pancakes always rise better... higher... the next day or two from the same or mixed additional batter. So my original question was what kind of mold is it that makes the pancakes rise so well and become so tasty? I assume this is just a time thing for the mix to get into gear. I don't know why it is, but saving some of the old mix for future mixes also seems to work for making them rise better and be lighter and tastier.

This isn't normal. Sounds like it developed some sort of mold. I would definitely dump it. Whatever waste is incurred is nothing compared to the risk of making yourself sick.