It is unclear whether one or two overflow pipes are leaking down the wall of this block of flats.
The green algae staining indicates the leak or leaks have been ongoing for some time. If left unattended the damp will soak through the wall spoiling the plaster and decorations inside.
As always, it is far better to keep on top of all those small maintenance jobs. If you ignore them it will cost you far more to put right in the long run.