Mold on the wall due to water damage?

Hello everyone,

we are renovating the house that we recently purchased. The owners pointed out to us during the second viewing that the drain in the shower had loosened some time ago (it is screwed), and as a result, water got into the masonry. There is still a small water stain visible in the utility room directly below the bathroom.
A few days ago we removed the wallpaper in the stairwell and saw something on the wall that looks like mold to us (see pictures). On the other side of the wall is the shower. Therefore, we suspect that the wall at this spot was once damp as well. The wallpaper was also somewhat loose, and the back side was partially red and black.

Question 1: Is this mold and if so, how can it be removed? I tried it yesterday with mold remover that we usually use for the grout in the shower (chlorine-based). As far as I could see yesterday with the wall still damp, it didn’t work very well.
Do you have any other ideas?

Question 2: The house does not belong to us yet (purchase price has not been paid and the land register entry is therefore missing), but we are already allowed to renovate. If it is mold: Does this count as a defect that still needs to be fixed by the owner? We were informed about it, but were not told that this wall was also damp, only the ceiling in the room below.

It is admittedly hard to see. I would think it is not mold but wallpaper paste or something similar. Supporting this is the fact that your mold remover does nothing. Mold is usually darker and would be more visible in the corner than on the surface.

Thank you for your opinion. Where you mention the corner: Only on the wallpaper (backside) from the corner were a few black spots visible, otherwise just something reddish like on the wall. The spot is also completely dry, but we do not want to simply wallpaper over it again without knowing what it is.

Well, these color stains look like old glue residues and/or wall paint. In old houses, you will surely always find a black dot here and there. If you want to be absolutely sure, paint the entire wall with an anti-mold product. It is available in 5l containers. But dryness is the best indication that nothing is happening there.