Wanted to ask about lens mold. I've never seen it, not even on a 70's vintage lens I have. What does it look like, how does it happen, and how do I avoid it?

It looks like a slight haze on the lens. It can progress to looking like fogging from condensation up to having depth and color.

Two separate phenomena are usually responsible. One is from improper storage/handling and moisture just collecting and having something starting to grow.

The other is a particular fungus actually that eats a certain vintage of coating applied to lenses that contain an organic component that is 'tasty' to the organism. This was not uncommon in lenses that started to appear during the late sixties and continued to as late as the early 1980's that had anti-glare and color correction coatings. The manufacturers changed formulations and this one disappeared except in vintage lenses.

Methyl Alcohol or Methylene Blue (organic microscopy stain) are the best remedies. However they may not prevent it from returning and usually this is going on everywhere in your lens. UV light in sufficient concentration will kill it, but the Alcohol is required to remove the waste product of the mold or fungus that produces the hazy stuff on a lens.

Old leather cases for lens should be discarded as they will harbor the organisms and expose the lens to sunlight for a few days every year, even just use them, to prevent or halt progress.

A lens with this problem needs to be totally disassembled and thoroughly cleaned with the above treatments.