ᐅ White discoloration on the wall appearing months after we painted it

Hello everyone, in 2018 we painted the lower third of the children's room brown. Now we have discovered a white layer there. The wall is an exterior wall and is quite cool. It can’t be efflorescence on the first floor, nor mold. The white layer is smooth, almost like a thin coat of translucent white paint was applied 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 touched the wall, there is no white layer...

It looks a bit like the wall is drying out there and pulling salts with it. Where the bed was placed, there might not be enough air circulation to evaporate or dry the area. It could be construction moisture or a leak. The wall doesn’t have to be damp on the inside.

Best regards