The car in question had a very serious leak, and because "reasons", it sat a long time (2 years) with wet interior, I mean it had a puddle inside...
The crazy thing that besides some rust on screw heads, the car is intact despite this, no rust whatsoever!
However, after stripping the inside, letting everything dry and going through the car with a wet vacuum, I noticed that deep inside the carpet, there are white spots that didn't go away no matter what I did to them.
What is this and how to get rid of it?
I suspect mold, but I tried some vinegar and it did not help. The white spots are quite rigid-ish, when poking with an old screwdriver, it takes quite some action to weaken one. Also, some of these spots are visible on the other side of the carpet too.
Other picture:
