That film is often the result of oils or fats from fish food (frozen, dried, etc.)
It does conflict with normal gas exchange. Often times not enough to cause a panic. But it can look bad, so that is a good enough reason to try and remove it.
I have found a few ways to remove this.
Slightly lower your water level so the return flow from your filter causes a ripple effect at the water surface. This keeps the film from forming, but it will reform if the water level rises back to normal.
When doing a partial water change, hold the hose about 1/4" inch from the water surface so that it creates a whirlpool effect and hopefully this causes the film " to go down the drain." This will be tough on/in a small tank though - hand gets in the way with tank decorations or not enough surface sludge is removed with ~1gal of water that has been drained.
Take a clean & dry (aka new) paper towel and lay it flat on the water surface and let it start to get wet. Wait a few seconds (make sure the towel does not sink into the tank!) and then grab one side of the towel and pull it across the water surface. Hopefully the oils are now stuck on the paper towel.
