Under the shade of a coolibah tree planted by Rolf Harris, we have billions of tiny blue-green algae. These algae are microscopic and most probably Microcystis (I haven't taken a water sample this year but this is the usual culpret).
There are at least three ways to look at this phenomenon.
For most people it's 'yuk'. Blue-green algae can be smelly, remove oxygen out of the water and kill fish, or produce chemicals (in this case microcystins) toxic to animals including ourselves. Simon Goodwin, who provided the two habit pictures, described it as the 'toxic death look like the creature is about emerge any minute
Another way to look at this is what a pretty colour. Even though the organism is a blue-green alga (a cyanobacterium), it looks bright green. That's the way algae are - their common names don't always help us identify them in the field.
The third way, is to celebrate this seasonal display of an Australian 'plant'. Who knows if it is really native to Australia but it occurs pretty much everywhere in the world now and has most likely been in this country as long as most plants.
I use the term plant a little tentatively. Only the 'green algae' are close to what we mostly call plants (flowering plants, conifers, ferns, mosses and so on). Others are variously distributed through the tree of life and more distantly related to green plants than animals like you and me.
The blue-green algae are nowhere near the green plants in our evolutionary tree and some people don't like calling them algae. They are in fact a kind of bacteria and they have a totally different physiology and chemistry to plants and animals.
Blue-green algae are the oldest photosynthetic organisms in the world and its thanks to them we have oxygen in the atmosphere at all. Their relatives ruled the world about 3.5 billion years ago I think (well, others think, but I presume).