Oh is that what it is? I'm not bothered about it and it doesn't even get in the way of focusing, my only worry was that it was some kind of fungus as I don't want it spreading to other bits in my camera bag. As it is not, I don't really care about it.

I'm a bit tempted to open it as there are 6 screws holding the whole thing together, but knowing me I'll probably end up leaving more dirt inside it...

What exactly is silvering? Is that the mirror surface deteriorating? If it can be repaired I might do it when I get the whole rig serviced sometime next year.