Definately mold. Molds propagate via spores & those spores can be released & float around when you disturb the colony, so you'll want to wipe as much of the stuff off both your bottles and your corks, using a solution of sanitizer (bleach, starsan, etc...) as you can BEFORE you start removing corks. The reason being that you could easily contaminate the contents of the bottle with these mold spores & not even know it as they're microscopic. Some molds are tough critters to get rid of & you might get one that can survive in an X% alcohol solution. That's my 2 cents worth, hope you find it useful, GF. EDIT: Forgot to mention that you might consider adding sulfites to protect against mold contamination. I know it would be a pain, but it's just a thought. Regards, GF.