I have a Lowe Aline Attack 50 pack from around 2000 and recently much of the inside has accumulated a sort of brown powder that is either, I suspect, some kind of mildew, or the remains of the decomposition of the pack's waterproofing.
The stuff is not easy to remove; if I beat the pack while holding it upside down, the dust gradually falls out. Wiping it down with a damp cloth or cleaner doesn't speed the process up much. The stuff appears to be only on the waterproof liner and, to a much lesser extent, facing surfaces. What is it? Can it be removed? Is it just time for a new pack?
Addendum-- I've already accepted an answer below but wanted to share that Lowe customer support concurs that the waterproof liner has decomposed.