Note sure what it is? But here is a link on the occluded gas bubbles on planchets. (wheat area examples on solid copper planchets)
http://www.error-ref.com/gas-bubbles/But not sure if it is this or not.
If it were damage to the coin, then the expected area would be above the date area on this over lay of the reverse:

But I don't see anything going on with the reverse on this coin. So I would keep it safe for now until we figure this one out.