I have a small safe that I keep some of my coins in and I thought that everything was o.k. Dessicant, ziploc bags, etc. However, I have notice that one silver proof in a PCGS holder has gotten what looks like fine white "dust" in the field. I was worried so I looked at my other holdered coins and saw that a Merc Dime in a NGC holder also has what looked like some corrosion on it's reverse. Imagine a dull steel penny, kind of whitish. It is a little visible looking "head on," but when angled correctly, it looks pretty bad. Any thoughts on what it might be? Both coins are in good holders, but something is definitely not right and I am worried that it may be where they are stored.