It's a vague term. It's likely to be internal, because most people will try to clean it off the surfaces they can reach. Fungus is splotchy unless it's really bad. It could be oil that vaporized from the focus grease and was deposited on the lenses. (That means it's probably on the aperture blades too.) It could be deteriorating lens coatings. It could be separating elements, where two elements were glued together and the glue breaks down. Element separation can be ruled out if you have an optical diagram of the lens and it doesn't have glued elements. The potential problems range from something easy as long as you can reach it, to difficult/impossible to fix.
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