How?

what's at the other side of the wall?

water is leaking through.

fix the wall from the other side

Yes. Agree with Bro. slimey. There is water seeping into your wall. The paint discoloration and mold growth confirms that. You'll need to find out where the water seepage source is. Fix it, then repaint the wall.

What should i do temporarily?

It would help if you can give more information on the wall. Interior? Exterior? Location ? (bathroom, bedroom, etc) Do you know what is behind the wall (i.e. are there piping? ) The room behind the wall? (bathroom, toilet, outside garden)

The watermark is obvious.

Behind white wall is a bathroom

1. bang wall.
2. fix the leak.
3. if no leak found = add water proof layer
4. seal up the wall.
5. repaint.

It will take some time for workers to come to fix the wall leakage, how should i clean the mold first now?

Water is seeping from behind the wall. Source unknown. Could be the piping, could be water from shower from the bathroom, could be other source. You will have to find out. For the meantime awaiting repair you can wipe down the wall with a wet cloth. The mold comes off easily. Be warn ya, leaks do not go away by itself. You will have to fix it. Else it will grow into a monster problem (electricity trips, damaging foundation, etc). If that wall is on the first floor (i.e. not ground floor) it becomes even more crucial.

The height of the leak starts from the top ... most likely a possibility of a wall shower head connection. Unscrew and roll some white tape to stop the leak.

As for the wall, use water and brush.

The amount of mold suggest piping leak.
Grout leak from tile lines leh..forever that spot bubbling paint but no mold

Pipe leaking, got solution for mold. But temporary la because still leaking.

Tarok wallpaper, one eye close, one eye open. Kautim.

The glue cannot stick to a wet wall.

There is no quick temporary solution.

It is bathroom but what is exactly behind the the wall. Is it a sink or toilet flush?

Toilet flush, shower and water tap

I meant at the most wet spot on the wall. It cannot be all three

Contractor will knock the wall at the most damp location as it is most likely the leak pipe location. Once fixed, will leave it and see it the wall dries up to confirm.

If you notice the picture, the leak starts from above human head when standing.

That would be the first place to check.

Investigate where is the leakage and then repair that first..

If you just repair this side of the wall, it will not be long term.