A closer picture of the area in question would help, but here's my thinking. Some foreign substance ( possibly moisture ) on the coin caused the area to discolor. If it has eaten into the coin, that's a problem. From what I can see, no big deal on a circulated common-date walker. If it was an expensive date though, it would be considered less desirable and would be discounted.
That's terminal stage toning, proper storage of coins is very important ... Paper, wood, chemicals, finger prints, air pollutants, moisture are all things that can effect a coins surfaces and there are many more...... It definitely hurts the eye appeal on this one, I wouldn't buy it personally unless it was for melt value
