I virtually never have surface scum:
Large water changes, active plant growth=> high O2 levels=> high bacterial growth rates which consume and decompose the protein and lipids on the water 's surface.
When plant growth is poor, the plants leak or proteins and lipids, the bacteria do not grow as fast, algae pops up, then you get scum.
Seen it a few times, took plant cuttings/leaves etc, this produced a nice oily layer.
Try this: put some excess plants in a bucket and leave it for a few days, you'll see an oily layer form.
As the plant rot/leach, the oil/promtins/lipids come out.
If you take a very actively growing tank, the O2 should be high enough to remove it and frequent waters with this will remove it.
Generally it's a sign I can further maximize my CO2 more when I see it.
Regards,
Tom Barr