Yes strange one this. There are some hits in Google but nothing too helpful.Hmm, not sure what that is now tbh.
Have you tried soaking up the water surface particles?
Easy to do, just use clean large kitchen paper towel or even sheet of plain A4 paper, lay the sheet of paper flat on top of water surface and let the paper soak up the surface water, it will take in the particles and containments too.
When paper just about to soak through, take paper out, and repeat with clean sheets of paper until satisfied you have taken out as much of this surface particles as possible.
Then see what happens after next water change and see if it returns.
An odd one really, hope the cleaning of water surface with paper does the trick.
I can soak the surface film but there are plenty floating around in the water column as well.
The floating particles don’t move or float to top so not Copepods or micro bubbles.
I will try daily water changes to see if this helps.