Damp walls

Probably a lack of ventilation.

Have you got a gas heater ??

Pictures would help.

Hola     you need old clothes   (splashes)    and some bleach to wash over the damp spots     (WC cleaner will work, try clear or lemon   pine may stain the wall green)

open windows         then spray / brush  the bleach         give it a while      then you may need to repeat      

when dry   (if the wall are still marked)    just buy some   damp  (bathroom/ kitchen)   paint       a roller or brush       try and keep windows open

please do not use a gas (bottle) type heater      they produce condensation     also almost banned from use   with other products of combustion that are a risk 

try also  (if you can)  treat the outside wall       with a type of     Ronson mason seal    (clear)     may be down to our Gota Fria  (storm)    last Wednesday  

ask at  Leroy Merlin    for the materials 

...or buy some mould remover spray from a DIY store and follow the instructions, but as Dave says, ventilation is the key.

Jeff