Hello, 

I need help with an unknown problem on my wall surface. It appears to have white patches/stains that resembles powder. 

I can't wipe it off using my finger, and it could only be removed through a damp rag. 

Please refer to the attached picture. From this, you could see that the water droplet actually cleans part of it, but the perimeter of the water droplet has the "white powdery substance".

Any advise on this? Thanks a million

Hey I had the same issue a couple of years back 

I wasn't convinced it was mould, i didn't see any mycelium and it definitely felt powdery. I believe is some kind of salt deposition...

you could use a damp rag with some diluted bleach and wipe down the wall (test on a small area first for colour fastness). if its mould the bleach will kill it and the white patches will not return.