Originally posted by Not a Number Fumigating with chlorine dioxide such as a Star Brite product should kill any mold/fungus and spores.
Powder may just be from deterioration/oxidation of the leatherette.
A cheap DIY solution is spraying it with an anti-mold liquid, then let it dry under the sun till it's completely dry.
I live in Thailand, and the last exceptionally strong rainy season has brought a widespread mold infestation (also affecting two lenses). Light (UV), low humidity, high or low ph (baking soda or bleach) are all enemies of fungi.
The anti-mold liquid can be made with 3 spoons of baking soda, 3 spoons of salt, and 3 spoons of hydrogen peroxide, diluted in 1 liter of water.
It leaves almost no traces after it dries.
A 5% solution of hydrogen peroxide can be used to clean a fungal infection from optics. You don't have to wipe, just leave the glass element a few seconds inside the liquid, fungus traces will disappear almost instantly
Cheers
Paolo