Grey Powdery Substance on Capsule

[ex]

Okay - An admittedly odd question... Over the summer I purchased a modest wine cooler (living in a small apartment in NYC I decided to buy a "Silhouette" from Danby (a 50 bottle unit).) So far I think it's been working well - constant temperature and humidity, etc. However, just in the past few weeks I noticed about 10 bottles on the top two shelves have developed a light grey powdery substance on the capsule (the metal covering the cork.) When I try brushing it off it is actually quite solidly affixed - using a damp cloth I can remove it but sometimes some of the color of the capsule itself comes off as well. The substance does not appear anywhere else on the bottle or on the label.

I have no idea what might be causing this - about a month ago I had not noticed anything like it. The only change I'm aware of is that from time to time the relative humidity in my apartment drops very low (I have a couple of humidifiers but they can run out of water pretty quickly when the heat is on and I'm out of town.) I realize this is not ideal for storage but the interior of the cooler appears to be maintaining a pretty constant humidity level of around 60% +/- 10% so I'm not too concerned.

Any suggestions on a) what can I do to prevent this? and b) is there any risk of harming the wine itself?

Thanks in advance!