Hen house problem

Hi can any one help. With all this damp weather I have today found a little mould on the inside of the hen house roof. What can I use to clean it/prevent it returing?

Thanks for the reply Foxy, will scub today (hope it stops raining). I think ventilation may be the issue so I will sort it and hopefully this will prevent it in the future.

OH left pop hole open last night and as they seemed happy with this we have done the same again tonight (the coop door is still closed tight and the run is also shut so they should be OK. I think they should be OK if I air it more during the day easier to do when not snowing or raining.

I think they will be well sheltered. The pop hole is facing a shed and has the composter/bushes/trees on the other side. It has the sun on it just about all day (when it shines) and it is always a nice place to be as it is tucked away. If they start to look ill or get a cold I will drill a few holes but I think it may have been my fault  so I want to to let it air as much as possible for a week and then if the temperature does not rise I will close the pop hole again. I usually find I've done something if it all goes wrong sometimes I try too hard.