Since it is green, it is probably verdigris. Verdigris doesn't normally come completely off just by dunking, in my experience, but it might help. Sometimes the stains are permanent. I'd try vinegar since that is what I use on brass for verdigris.
Is it worth trying to rehab? It looks really really green in some areas, I'm a bit hesitant and thinking of just passing on it. It's $30 which is on the high end of what I'm willing to pay for a bag that needs work.