Dampness on a summer house-What would you suggest?

[visionexplorer]

I aked this question on reddit but it was suggested I sign up to this forum as there are far more experts on here so here goes!

I've been asked to paint the outside of an elderly relative's summer house .It looks to me like it's never been painted and appears to have some kind of dampness or mould on the outside of it. (These images are from the outside,the inside appears fine)

Should I just paint over it or should I give it a wash with bleach or something first?

My only concern is doing even more harm to the wood than has already been done.

Thanks