I am scanning some of my old negatives and slides. On some I have found a weird blotching, almost like a fungus that is growing on one side of the slide/negative (I've found the same on both media). It is really obvious and seems to be on the shiny (?) side, it is easily observable from several feet away without holding up to the light when looking at that side of the negative, but cannot see from the other side until you hold it up to the light and look closely. I'm not talking about little splotches, these are big and all over the negative/slide, essentially ruining them. See attached photo.
Any idea what this is? I am assuming these are not salvageable. These have been in archival preserver sheets in a binder for decades.