"Green rot" is
verdigris which is either copper carbonate or copper chloride, depending on environmental exposure. This looks like verdigris to me, where it's corroded into the coin, leaving a raised circular lump. Judging by the reddish coloration, this penny was exposed to something that reacted with the copper--perhaps mildly acidic conditions. 1951 is a pretty common year, so the verdigris doesn't affect value that much for a VF coin. When I find some "stretched pennies", I'll include a nicer example from around this year with those in a trade.
