Purple discolorations on plaster on brick wall

Hello,

I am currently renovating my living room - my wife and baby are eagerly waiting for the retreat space - and now I have discovered purple discolorations under an old wooden wall.

The discoloration also appears on a similar wall (very thin plaster on a brick wall) in a barn here on the former farm.

The color seems to be of a more mineral nature, so perhaps an efflorescence?

I am happy to provide more details; unfortunately, the people on site cannot help me.

See photo

I am now wondering if I can safely remove it and then plaster freshly, or if it needs to be treated with V12 or something similar before removal.

I would be very grateful if someone could help me with advice and experience.

Long searching on the internet, both in English and German, was unfortunately fruitless.

Where can this discoloration be found? Only in corners?

Where is this discoloration found? Only in corners?

No, especially in the middle of the wall at 60-100cm height.

Similar in the old barn (outbuilding).

I currently do not see the edit button, therefore an addition: the former farmhouse was built around 1890. The discoloration is each on the inside of exterior walls made of natural stone/rubble stone (basalt/granite) and about 48-60 cm thick. On the inside of the exterior walls, however, bricks are installed.

The first thing that comes to mind spontaneously is: "Iodine stains starch blue," i.e. I could imagine that components of [Viehstallluft] reacted with the plaster under airtight conditions and in the dark in a way that is not known from the unboarded [Viehstall]. According to the picture, it concerns spots behind the ventilation in the same way as spots behind the former battens.