I have here a silver-oxide button-cell SR626-size watch battery. It's still in the package. There's no well-known brand name printed on it. My dad bought the battery from the store 12 years ago, based on the attached receipt. I haven't voltmetered the battery yet.
The top of the battery says "+". It looks like it's in fairly good condition.
The bottom of the battery is lightly covered with a whitish or very pale greenish substance.
I've tried various Google searches, including [ silver-oxide battery shelf life ], and haven't found answers to my questions:
What could the mystery substance be?
Is the battery likely to be still good?