In article ,
Saeed writes:

I had a damp problem in my kitchen, cause unknown but contributing
factors may have been:

In 5 weeks this has made a big difference, with the kitchen walls now
almost completely dry.

On one part of the wall, I hacked off some loose top layer plaster to
leave the browning undercoat. The undercoat was also damp, but dry now.
I have however noticed on this now dry browning a white powdery deposit,
and I'm not sure how to read this. Is it a sign of fungal rotting, or is
it something harmless that can just be swept away?

It's due to moisture moving through the wall and evaporating at
the surface, leaving behind the salts it was carrying. Brush it
off and if it reforms, you still have moisture coming through the
wall.

--
Andrew Gabriel