We built a shed for my son. It was damp coursed and plastered and has a tiled roof. Its been painted and has electricity connected... But now, every morning, there is milldew on the walls - How can we stop this from happening?

You are lacking in answers due to your lack of info. Where do you live? Is it below grade? Bathroom or all "walls"? What type of foudation, ext walls and heating/ ventilation system?

You are implying that mildew cleaned off the walls grows back the next morning which would be an astonishing feat of nature and worthy of a scientific grant and study.

I live in Ireland so it is damp all the time. The shed has double glazed windows and only has a super ser for heating. It's not used every day so therefore its not heated every day.
I didn't build the shed so I'm afraid I don't know what below grade means.. I do know that the foundation was dug about 4 ft down and filled with cement.
The inside walls are plastered and painted and the outside is pebble dashed.
Maybe I used the wrong word when I said 'milldew' there is black fungus looking stuff on the walls that wipes off, but comes back again fairly quickly.
Where can I go for the grant? (jk)