Hello Akai Ryu,
If the wall is showing signs of mould growth, chances are the insulation is sweating. Other possibilities include gas leak, dirty fancoils, both resulting in condensation and icing which could also contribute to moulding.
As your pipings are concealed, it would be more tricky to ascertain if the mould is due to sweating insulation or pin point the exact location of the leak as water flows with gradient. You would most likely need to have your contractor measure the gas pressure to ensure there is no gas leak and also check the conditions of the fan coils. If the gas pressure is fine and the coils are clean, then chances are the insulation is the problem.
Best that you have your contractor come up for a check for further advice.
Best Regards,
Jason Tham