It is suspected that the heating pipe leaks and the wall skin falls off and turns black.

As long as it is moldy, there is only one reason for getting wet. Now it's time to determine where the water leaked in. If you are already wet when you decorate, and the heating pipe has not been connected at that time, it should be that the water supply pipe in the bathroom may leak, but it can only be said that it is possible, not necessarily. I once had a problem when I was working as a staff canteen on the 7th floor of the top floor of a group company. A corner is always wet and putty comes out. However, no water pipes pass through the wall and the nearby floor. Finally, it was found to be the bathroom on the fifth floor, and the water supply pipe leaked. As a result, bricks and concrete will absorb moisture until the seventh floor! After the bathroom on the fifth floor was repaired, three days later, the seventh floor was no longer wet. Another time was in the penthouse on the eighth floor. The decoration on the sixth floor exploded, and even the wallpaper on the eighth floor absorbed moisture and turned yellow! Since your door cover is moldy, change it anyway. It will be easier to judge if you take it apart and remove the wall near the aisle.