Asita,
I just got a very detailed and helpful response from book conservator, Doug Sanders. He looked at the pictures and while it could be mold he suspects it is probably a “soluble or finely powdered grit (drywall, plaster, clay…) that may have migrated inward and dried there.”
He then suggests to “brush off the deposit in a fume hood, or in a HEPA vacuum (on low suction of course).”
Then place it in a four-flap enclosure and isolate from the main collection.
Sanders also suggested flagging the item to be available to patrons only by librarian/curator approval. This way the patron is aware that there is an unknown substance on the book.
I hope this helps!