for at least 3 of the last decades I've been aware of the issue of mold attacking the optical surface of things like lenses and CD's. I have been in the past involved with a few discussions, as well as published a little information about the topic on my blog (here).
The other night I pulled out a CD from my collection to play which had remained untouched there for some years (probably at least 10). While it was in its case, it had not been opened (although had been moved and even packed away in boxes while I was in Finland for a few years) and was in generally good condition.
The fungus I spotted (perhaps many would not have even spotted it) was so good a sample I just had to take this series of images to show you just what can grow on your lenses (and CD's) and why you should take seriously
You may just be able to see it as a small speck to the right of the pen cap clip.
getting in closer you can see it more clearly ... and what a classic specimine it is.
lastly a crop from the center of that image above showing the fungi "mycelium", and the fruiting body in the middle.
Aside from the fact that it's a speck on your lens you really should do something about this as they do actually etch into the glass surface (or plastic).
An excellent image of this can be found here and here where great examples of photographing the after effects of the mold on the surface of glass.
so my advice to you is that if you live in a humid climate (near the sea for instance) then to keep your valuable lenses in a dry container and perferably one which has been made humidity free.
As you will see (reading my above reference to my 2008 post) using silica gel is only patting yourself on the head and has no effect. Here is (reproduced) the graph of how the humidity went in a sealed container with some high grade indicating silica gel in the jar.
Figure 1: humidity measurements
Lastly don't leave your camera in its case; that is a sure fire way to get the best possible fungus infection you can (short of leaving it in the bathroom or in a damp room in the basment).
If you are interested in some more information on this subject, try reading this article of mine.