Open Bordeaux wines rather frequently and you'll grow very accustomed to mold on the top of the cork.
The foil that is put over the top of the bottle is not necessarily airproof or moisture proof and so a near perfect formula for mold to grow.
The fact is, I find it quite charming. I actually am a little disappointed when I don’t find mold; it just looks so romantically rustic, like you’re about to imbibe some ancient potion hidden away for centuries until you discovered it.
In over 25 years of wine study, I have yet to open a single bottle with such mold that was negatively affected.
At any rate, even if the wine had been “corked,” a reputable dealer would gladly receive the wine back opened, tasted etc.
For some reason though, in more recent years, I have had far fewer wines present with this “attractive” (in my opinion) phenomenon. I suspect that was for a couple of reasons.
First being that my consumption of French wine (until recently) has been quite low due to France’s wretched attitude and actions toward our country. They have since gained some sanity and are behaving somewhat civilly these days.
But I suspect that the technology of placing foils on bottles has improved as well as the use of synthetic corks gaining a little steam even in France.
All in all, mold on the cork means nothing so, as “Billy” explained, wipe the mouth of the bottle clean and raise a glass!