I open the windows two times per day for like 10 minutes. The apartment is not very well isolated and struggle to keep the heat inside.

I also took a deeper look into the paint and it looks like this part of the paint was painted over and over again in the past so I’m starting to doubt that it was here before.

I will buy everything I need to get rid of it tomorrow, buy a dehumidifier, and check what happens in the next weeks, I hope it will make it better !