Improper filtration and seasonal algae bloom can create a film on the surface of a fish pond. By draining at least two thirds of the existing water and adding fresh water along with cleaning your filter to aid in increased circulation should clear up the surface film in a few days.
Having a circulating filter helps. If you don't have a filter you could try using a spoon and just barely push it down under the surface of the water to skime the film off. Another trick is to use paper towels and gently drag it across the top.