The water in a swimming pool needs to be clean and free of any contaminants if it is to be inviting. A greasy film lying on the surface of the water is the antitheses of that quality. Get rid of the greasy film that has beset your pool due to an insufficient free chlorine level. You will need to sanitize the pool's water using chemicals acquired from a pool/spa supply store or a home and garden center.
Measure the pH level of the water, using a pH testing kit. Write down the test results on a sheet of paper. Use the instructions that came with the pH testing kit to determine how much chlorine per gallon you will need to reach a safe level in the pool.
Write down the total amount of gallons of water in the pool. You can get this figure from the contractors who built the in-ground pool, or from the manual that came with your above-ground pool.
Multiply the total gallons of water in the pool by the chlorine per gallon number you wrote down in Step 1 to arrive at how much chlorine "shock" liquid is needed to kill the contamination that is making the surface of the pool greasy.
Stand at the deep end of the pool. Pour the chlorine into the pool as you walk around the perimeter. Continue walking around the perimeter as you empty the contents of the chlorine container into the pool. Let the pool's filtration system circulate the water for a day to remove the contamination and bring the pool's pH levels back in balance.