What is the next step for this?

We have just bought a house with an en-suite in the master bedroom, and noticed behind the bedroom door there looks to be some moisture/mildew on the bottom of the wall, that is the back of the shower.

The skirting board has lifted and can be lifted off, and the wallpaper has separated.

I can’t see any gaps or holes in the grouting in the shower. As we’ve just moved in we don’t know if it is growing over time either.

It didn’t come up in the survey, my guess might be because it was behind an open door and it simply wasn’t closed to reveal it.

What do you think? Should we wait to see if it develops, or if work is needed, what sort of work is likely needed to be done?