Damp/mould issue

I have a question about an old loft conversion in a property we bought.there appears to be a dark mould growing on the side of one of the main beams with runs the length of the loft conversion ceiling. The beam was painted white by the previous owner and I imagine it must form part of the main structure of the roof. Is this likely to be an issue with the roof i.e slate or could it be an insulation problem (if there is any) I can't see as it's all been overboarded. Any advise would be appreciated.