How often should I moss treat my roof? Why?   

Effect of moss, mould and lichen on iron roofs.
The roots of moss, mould and lichen slowly erode through the paint surface of the iron until the roots are sitting on bare iron, which rapidly rusts. This treatment prolongs the life of roofs by many years and needs to be repeated every 2 years.

  What are bleeding nails?

There is a tendency for an iron roof to lift each time the wind blows over it (vacuum forms underneath). As time progresses this constant lifting effect forces the nails up and the roof becomes loose; the constant pumping effect of the iron running up and down on the nails causes the roof to leak -  the term is bleeding nails. The iron needs to be made secure but you cannot just hammer down the nails that are sticking up they will just come out again; they need to be replaced with a larger nail, in this case your 60mm nail with a smooth surface would be replaced with a 75mm nail with a twist thread. This nails are also a little fatter, so even in the old hole, the grip is tight.