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As well as motion from convection there is also the fact that the scum is insoluble, this means that attractive forces between 'scum' and other 'scum' are stronger than those between the scum and the water so there will be a tendency for it to congregate as electrostatic attractions have an infinite range.

I'll take a wild guess based on what it looks like I'm seeing in the video.

As water evaporates from the surface, it leaves a film of mineral deposits on the surface which grows thicker and heavier until it starts to sink where the thickest parts are.  The parts that sink instantly re-dissolve.  It's also possible there's a very thin film of vapour between the liquid and the mineral layer breventing it from coming into contact with the water until it starts to sink.

I'm probably wrong, because I have very little knowledge of this sort of thing, but that's what appears to be happening in the video.