Efflorescence on interior wall after leak fixed

[humanracer]

I had a roof which had probably been leaking for around a year and got worse. It caused a corner of the kitchen wall to become damp and wet. This was during the renovation of a kitchen so the walls were not plastered. I got the leak fixed (broken tile) and there were no more leaks after heavy rain. I wanted the kitchen finished quickly so I got it plastered but noticed in that corner the wall was still wet. I kept drying it within a heater and airing the room. Then, perhaps too soon, I got it painted. There are some stains more worrying white salts which I believe to be Efflorescence.
I don't want to have to replaster this whole corner. What's the solution? Rent a dehumidifier?

 

[Brigade77]

https://www.screwfix.com/c/heating-plumbing/dehumidifiers/cat840500

 

[humanracer]

So I took some paint and plaster off one part of the wall and it’s dry now however the walls above it has the white powder dripping. Remove the paint? Use some kind of breathable plaster?