That's the thing. The house was poorly constructed, and try as I might, I cannot identify any issues with leaking. there are no water marks, but mold will grow on one of the walls, and this part is in the area that is poorly ventilated. I installed a ventilation fan, but didn't solve my problem.

Ozak suggested the dehumidifier crystals (humid hippo) but this is not a small area. Room size is about300sqft, so will need a lot of crystals and frequent change. It is also not as effective to remove the damp musty smell.

If I turn it on when I'm out working, and turn it off when I get home, then turn on the air conditioning instead, feels like not so energy efficient. The aircond will take ages to cool the room and use more energy.

Haih...what a dilemma 

On top of the wall is the top floor balcony~retiled and waterproofed

Behind the wall is my neighbour's wall. They have a problem with mildew in the whole room, not just that wall. And I believe once I move in and start using the air cond regularly, I too will have the same problem
Yeah, same here. That wall is exactly where my built in closet is
The wall is back to back with neighbour's wall. Contractor saw before Reno. He also shake head dunno what to do