To be more specific, waxes and oils migrate, and lift salts with them. The heavier and deeper the additives, including waxes, the more shows on the surface as volatiles evaporate, leaving crystalline paraffin or waxes, along with salts lifted from leather.

I once had the super genius brainstorm of super-saturating the leather to remove all vegetable tan acid salts (ie oak tanning), but all I received for effort were sheaths which turned even whiter and brighter sooner, along with petrified forever leather...still corroded the snot out of rivets and knives.

If you want acid free, you use heavy oil tanned (actually mostly done with an alum salt) which has between very little and zero reaction to metals. Eventually, the acids left in leather will destroy the fibers, no matter what you do, when using vegetable tanning.

Welcome to white sheath land. Wipe it off or rub it in, makes no difference, treat as salt stained boots.