We have a naturally occurring plankton bloom on the west coast of Australia, when water surface conditions are right. I believe its' also on the east coast,  I've seen it offshore from Sydney, from an aircraft.  It usually occurs during warm weather and reasonably calm conditions. It varies from just a thin film to a very thick scum, which can wash ashore and coat the beach,smells like goldfish food as it dries out and turns purple. You can actually see the tiny individual clumps if you break it up by stirring the water. And it sits mostly right on the surface, doesn't extend down the water column. Your video looks more like a stain in the water, like a tannin stain? The sort of thing you see in a river outflow coming off heavily vegetated catchment, commonly referred to as "tea-stained" The way it seemed to cut off on a current line, and stained the water below the immediate surface,  makes it look like that?   The stuff I was referring to over here doesn't hurt fishing at all, IMO.

When your video started, in the ute, I thought I was looking at Brett from Flight of the Conchords  Sorry, couldn't resist that.

 Always wondered what is exactly those conditions? We get horrible green scum often referred to as natural algal blooms on the metro coast and interestingly the health dept say that excesive nutrients cause large algal blooms. Would explain the link between the WA Water waste water being pumped off Scarborough, City Beach and Point Peron.

Brett from Flight of the Conchords you think...? I will take that :) haha

  Yeah one would think it would have to be excess nutrients in the water plus sunlight, and maybe certain water temps? to make the algae grow. Definately wasn't tannin staining, it didn't show up that well in the video but it got very red at one point.