Video one shows normal behaviour of males. I say normal because this is how a male fish of many species behaves, or similar. But watching the video the sunset Platy
does seem to be a bit ill-tempered, and I would be somewhat concerned for the other two red Platy
not being able to escape his aggression sufficiently to avoid stress.
I don't usually worry too much about this sort of thing when it is within the species, and is natural behaviour. I rather assume the species can deal with it. When it spreads--if it spreads--to other species, that is more serious and the offender must be removed. But I don't know how much of this aggression the red Platy
can manage to handle as natural before it begins to affect them negatively.
The second video shows flashing, and likely due to ich (white spot). Any parasite will cause the same, and ammonia/nitrite sometimes does, or other issues with the water. But most often it is ich. I see it in a couple of my tanks now and then.
Ich is present in all aquatic systems, always. The old wives' tale about introducing it and then miraculously eradicating from the aquarium with treatments is just that--false. Ich is present in every aquatic system on this planet, as far as I know. But you rarely see it on fish as spots. Reason? The fish build up an immunity. But it does now and then get in the gills, which is why the fish flashes, trying to dislodge it.
The fish's slime coat protects them from ich, and it also contains various chemical barriers that may function here too. It is only when the fish is stressed--and for those who read my article on stress, you will remember that any stress causes a deterioration in the fish's slime coat--that ich is able to get through the defense and we see it as spots. Usually on the fins first as they are less protected.
This is why you can have an aquarium that has been running fine for months and years, but if the heater fails and the temp drops, you have a major outbreak of ich in no time. It is always there, waiting for the chance of infecting the fish when they are weakened by stress.
So Simon, don't worry. Maintain good water stability, avoid stress as much as you can, and the fish will stay healthy and fight this off.