Decor27

If it's dusty and it's got a black mold on it it could be dangerous to your health.
there is a product on the market called peel stop
which would stop the flaking of the paint apply a coat of this and then two of your top coat hope this helps .
chris decor 27

Rub the roller marks down with some medium sandpaper until they are flat against the surface and then repaint the surface with your chosen top coat

Without having a closer look it sounds like it's pva glue that has run down from when the plaster out was applying the pva glue to the walls you will have to put some sort of stain block on bin zinsser he's a great choice initial block out the dark lines and then apply a couple of coats of your chosen top coat hope this helps.
chris
decor. 27

Every few years it's good to paint the render of your house to keep it water tight.
painting every year will be a bit excessive i suggest every 3 to 4 years hope this helps
chris
decor 27