Algae is an infestation that causes all kinds of problems in a swimming pool, from the gross feel to the ruined appearance even to potential health affects. Algae can also be hard to kill, but fortunately almost all treatments are inexpensive--although they may be time-consuming.
One of the cheapest ways to deal with pool algae is to keep chlorine levels high enough so it doesn't grow. A chlorine test kit can be purchased to make sure levels are high enough to kill algae but not so high as to damage skin or bleach hair or swimming suits. A good pH level for killing algae is 7.2. Algicides can also be used to kill algae already present; follow the directions on the package.
Clean the pool on a regular basis. At least once a week, the walls and floor of the pool should be brushed to prevent algae from sticking. If they start to feel slimy, algae is growing. It's also important to keep pool filters clean. Sand filters should be backwashed and cartridge filters should be removed and rinsed off with a hose. Make sure they're working properly and sucking in any dirt and debris that falls into the pool. Keep the surface of the pool skimmed.
Even if your chlorine levels are fine, the water must circulate to prevent algae. Make sure that you have a properly working pump that's circulating the water throughout your pool. Vacuum filtration systems will also help. If you don't have a pump or it isn't working correctly, make sure to stir the pool every day, particularly if you have used algae-killing chemicals.
Other Prevention Tips
Algae can sometimes attach to pool toys and equipment. Remove it all from the pool and wash with a high-pressure spray, then disinfect the equipment with a bleach cleaner and allow to dry before returning to the pool--but only after the algae is gone. Shock treatments work really well at eliminating algae; if you notice algae is still present after one shock treatment, try another with twice the recommended amount.