ᐅ White substance on the wallpaper – what could it be?

Hello,

I am new here and have no experience with house construction or installations (my main expertise is computers and networks ).

Now I would like to know what these white spots on my wallpaper under the window could be.
Is it some kind of mold or is it "efflorescence"?

The material is very soft and dry, like powder.
It "crumbles" or produces dust as soon as you touch it with anything.

If I have broken any rules here, I apologize and will correct my post if necessary.

As you describe it, in my opinion this indicates lime. The walls urgently need renovation. It would be advisable to measure the moisture levels in the rooms.

Saltpeter/mold, toxic!

Hello roof inhabitants,

the white spots could indicate water damage. If I were you, I would have it thoroughly checked!

Thank you very much for the responses!
I have already hired a professional (roofer/master painter) to take a look at it.
It will be addressed right away, and at the moment, the attic apartment is vacant or unoccupied anyway.