That film is probably body oils, hair conditioner, sun tan lotion, sun screen, fabric softer . . . all the things that wash off when people get in the water. You can try to put a scum bug or scum ball inside the skimmer basket. These are available at pool stores and at amazon.com. They absorb oils as the water runs over them in the skimmer and will turn darkish brown after a while. Rinse them out, put them back in.