1993 Cents are Copper Plated Zinc What you have is a plating issue. Eventually the Zinc at the D will be exposed and you will have Zinc Rot. Zinc Deterioration - http://www.error-ref.com/zinc-deterioration-on-lincoln-cents/
Nice magnification but it's hard to tell without natural lighting. More than likely, it's due to environmental damage that caused the mm area to blister. I bet some of the stuff near the rim is due to corrosion as well.
I hear you guys, and do understand the zinc. But as I look just dose not look like rot. Also if you zoom in at the inside bottom of the D. It looks like a tail of a letter or something.
The Denver Mint does not use any other letters for their Mint Mark. This is only Zinc Deterioration and nothing else. We have seen many Cents such as your here on CoinTalk.. not an error, only damage.
You can see the corrosion near the rim. Once the copper shell is breached, the zinc is going to react. Depending on the reactant, the zinc compounds can expand ). As more zinc is exposed under the plating, the copper plating will bubble (think of rust on a painted car body). That's what is happening here
