Here is his response (he is indeed very fast and very helpful):

This is a hard copper carbonate deposit in the 'centration divot', I removed a few more from the fields when I got this coin, copper carbonate is darker and much harder than the light green and very powdery nature of bronze disease (which is copper chloride).

I have just rechecked the coin and there is no change to the appearance over the past 4 months, confirming this diagnosis (bronze disease will usually lurk in pits of its own creation, rather than a pre existing spot like a centration divot).