Hmmm? 99% probable its salt ;
1.wet index finger in mouth...
2. rub on small part of white "stain"
3. taste finger.
.. if its salty, be sure there is no odor of urea
(odor indicates cat pee
4 no odor of urea, it's probably salt!
try diluting/dissolving some with a wet sponge in a small area. if that works youve got an answer...