You would want to create some water movement at the surface. I use my filter outlet to create surface agitation by having discharging the water from the pump right at the water surface. An airstone will also do it for you.
I’ve had this same issue, except I have an airstone. It looks like a rainbowy substance, kind of like gasoline.
You may also want to consider more frequent or larger water changes to get rid of the protein.
Soak up oily films with a paper towel. If you notice a milky or oily film on top of the water, you have protein build-up in your tank. This is normal, but you will not be able to remove it with a net. Instead, take a single piece of a paper towel. Lay it on the surface of the water until it is soaked through. Remove the paper towel. The protein will come off with the towel. Repeat with fresh towels until the surface is clear
A surface skimmer also does the job wonderfully as well!