White coloring of the wall after we painted (months ago)

Hello everyone, in 2018 we painted the children's room (lower third) brown. Now we have discovered a white layer there. The wall is an exterior wall and quite cool. 1st floor, it cannot be saltpeter, nor mold, the white layer is smooth, it almost looks as if someone had painted a semi-transparent white paint very thinly over it. It is smooth and can be wiped off. The plaster underneath is dry and warmer than the cold wallpaper....... we are puzzled....... The bed was against the wall and where the bed was against the wall, the white layer is not.......

Looks a bit like the wall is drying out there and pulling the salts along. Where the bed stood, it may not dry out due to the lack of convection. Either construction moisture or there is a leak. The wall does not have to be damp on the inside.

lg