What could be causing dampness in a bathroom wall?
If it’s a wall in a high rise, I doubt if it’s a roof leak. 

I’d guess 1) If there is a sink, toilet, etc. on the other side of the wall I’d check for  a loose pipe , or 2) I’d check for ventilation/condensation problem. 

1) The leak might be from a loose drainline that only leaks when it’s really really full. That is to say, it could take years to produce enough water to create that mold. Fill up the sink/tub and see what happens. 

2) If your exhaust fan duct is not insulated, it could create a lot of condensation which can accumulate and run to the floor. (However, I don’t see mold streaks, so I doubt it could be that...)