Indeed, in my experience, there's some kind of biofilm that forms on the disk surface. Doesn't look like anything more than a thin stain or discoloration, but disks so afflicted never read correctly. Suspect that whatever it is, it's between the mylar and the oxide layer so the bits literally flake off when the head hits those patches.

Why do I think it's biological -- just the shapes of the patches resemble what you see in laboratory petri dishes, and that the amount of such corruption seems to correlate to humidity. Disks from an attic seldom have this issue, compared to ones from basements.

The insides of the sleeve were lined with felt to cushion the disk and allow it to slide easily. It also unfortunately attracted moisture and could promote mold growth in very humid environments.

It also means if you drop it in water (as I once did) the felt will expand then be dragged along with the disk and literally crash into the head if you were foolish enough to put it in a drive. (Which I fortunately didn't.) Drying it out is not simple. But for the 3.5" types opening the shell is. You can then transfer the disk into a donor shell after giving it a good wipe with lint-free cloth and be on your way.

For dealing with mold on the disk itself, a brief Duck search reminds me that UV light kills most biologics. As does ozone. That will at least stop it from growing.