Sounds like you’ve got a book that has…mildew. Pretty common. Solutions - well, as a book collector, I checked into this with a very reputable rare-book dealer (Quill & Brush, out of Maryland - they “wrote the book” on collecting first editions)and they said the solution is, if you can believe it, kitty litter. Here’s the recipe:
Get some standard clay (i.e., non clumping) kitty litter
Put it in a pan deep enough to allow you to bury the book in litter (I can hardly wait to hear the sarcastic comments on that one). By the way, if it has a dust jacket, take it off before submerging…
Leave the book in the kitty litter for 3 weeks (!) The clay neutralizes the mildew
After 3 weeks, take the book out of the litter,brush off the dust (this part scared me - I assumed the dust would stick into the cloth of the binding, but it came off easily)and put the dj back on.
I have tried this on two books and it did fine. Best of Luck