Floating oils, dirt & wastes can combine to form a scum line around the pool; this is why tile, an easily cleanable surface, is placed at water level around the perimeter of the pool. There are many tile cleanser products available which are applied with a scrubbing pad or brush and a little elbow grease. Abrasive cleaners work well, but should be avoided in vinyl lined pools, or pools using products such as Baquacil. On vinyl pools use a vinyl cleaner such as Armor All Cleaner (not conditioner), and on Baquacil treated pools, use a cleanser made without chlorine.
Cleaning the scum inside of the skimmer frequently will help to keep the tile cleaner, as scum sticks itself to clean plastic. Using enzyme products can reduce or eliminate the amount of attention to the scum line as the work to "eat" scum producing substances.