Salting on the chimney

Hello everyone,

attached are a few photos of my fireplace on the upper floor and the pantry through which the fireplace runs.
Who can tell me where the efflorescence might come from and how I can best get rid of it?

The upper floor is unplastered; the white plastered fireplace is in the pantry below. The white paint is already blistering.

The house was built in 1978.
Thank you very much

Hello, what does the chimney above the roof look like?
For me, it was built with clinker bricks, which means these were pre-made attachments, and in the attic, it looked similar to the 4th picture but it was damp and that was easy to fix: formwork, lead connection, Delta foil, slate, zinc cover – at least that’s how it was for me. But if that’s not moisture in the chimney, then I’m also puzzled, although it at least looks like there was moisture in it at some point.

Regards, abo68