Shaun is spot on.
Modern housing and enforced lifestyles often clash to cause mould growth. The designing out of draughts and heat loss has also designed out ventilation. Fear of crime, and heat loss, means that leaving windows open is not common.
If you examined the amount of moisture respired out through an average night you would understand why condensation is such a problem.
That said, there should be a proper examination of the walls concerned to establish if there is any structural issue, or any water penetration that is causing the problem.
I once lived for several years arguing with the landlord that my 'lifestyle condensation' was water penetration. This was eventually found to be the case, but it took persistance and the intervention of the Building Research Establishment to convince them of the case. The outcome was demolition, so it definately was a little more serious than condensation!
I hope that you follow Shaun's advice and that a satisfactory outcome. Meanwhile some tips
Do not use any liquid to clean off the mould, not even bleach. There are products you can buy, but even these may only feed the mould (I used one that changed the black mould to red - it was a change but not a cure). Mould is very robust and breeds when wet. Choose a dry cool day to wipe off the surface mould with a dry cloth (wear a face mask and ventilate the room so the spores are not inhaled, and hopefully are removed). Keep furniture away from outside walls allowing airflow to minimise the chance of damp forming. Remove the wallpaper as this may trap moisture and provide a root for the mould.
Once again, good luck.