Deck maintenance - general question

I would like to understand why I have to pressure wash a deck and what happens if I don't.

We have treated pine. It was once upon a time painted with something, but that is long gone, and the wood has acquired a gray hue. I power washed a lot of it today and yesterday, and that takes off most of the gray layer. However, there are also lots of dark green areas, which I assume are algae or molds/mildews. It is very hard to remove them - I use a bristly brush and it comes off as a greenish liquid.

Since I have done so much of it already and since I also already have some deck paint, I will finish the job at least in the exposed areas.

If I did not do any of this, what would happen?
Is there a better method (non-toxic) to remove the green stuff?
How often should one (if one should) wash and/or treat the deck?
No matter what I do, there is green stuff between the boards, will that "eat" into the wood, even if the top is painted?

Obviously, I am trying to get out of this awful job, but on the other hand, I also want to be sure that I am not loosing value if I turn lazy?

DH is much more lackadaisical than me, I want to do it right, if I have to do it.

Your ideas will be much appreciated.

C.