Amadeus

In my en suite I have a skylight - a velux window.
The en suite has a fan for Shen the shower is on which I think works ok. However it looks as if there is a patch of mould on the inside of the frame. The frame looks untreated - no varnish or anything but I don't know if the damage is caused by water from the inside or outside.
Not really sure what to do here - sand it down, varnish (?), and see what happens over the next year? Or get a man in yo look at it from the outside and try and ascertain where the problem is. Any suggestions?

Thanks