Asking owners of the 1st generation IS super telephotos for your opinions on the material of the barrel wall and how it looks.

When looking into my 500 f/4 L IS 1st gen last night I noticed the barrel is outlined with some sort of felt fabric, I assume it is similiar material used in the interior of lens hoods. When I shine my flashlight into the barrel of the lens this material looks fuzzy and has the silverist-aluminum color, (sorry can't find a better way to explain this). At first I though it was fungi that are growing but it doesn't look anything like fungus from what I saw online and also the elements are all clean and I can't even spot a spect of dust. I live in a very dry enviroment of Southern California, my indoor humidity rarely ever go above 40%.

Sorry for posting too many threads the last few days, recently caught the photography bug and wanted to give this thing another try. My gears have been rarely used the last 5-7 years, just wanted to make sure they're still in good condition, my 13 year old 500 f/4 L IS has always been my pride and joy.