A film on top of the water in a swimming pool can be the result of several things. A lack of water circulation may be creating stagnant water. Or, dust, pollen and other air pollutants may be settling in the water, requiring pool skimming and filtering. In rare cases a film can represent a problem in water chemistry.
Test the water first to ensure the chemicals are properly balanced, using a water testing kit or test strips. Take measurements for pH, alkalinity and total chlorine and add chemicals as needed. This will vary depending on the reading of the testing kit.
Turn the filter on. Empty skimmer baskets to guarantee there is not blockage. Let the filter run for at least 24 hours and see if there is an improvement. This will solve most cases of filmy, slimy water.
Skim the top of the pool for debris that may have settled on the pool surface. Add one pound of diluted granular shock to the water for every 7,000 gallons of pool water. Run the filter for 24 hours and retest the water. The slimy looking film can be an early onset of algae.