Jan's right. I suspect your CYA / Stabilizer level is 0 (check the pool store readout). You're going to need lots of chlorine to fight and kill the algae. Your best bet is either bleach, or, if you have it available, 12% Liquid Chlorine. On gallon of 6% bleach will add about 1.7ppm of chlorine to your pool. A gallon of the Liquid Chlorine will add about 3.4.

If the pool is concrete, tile or other similar material, you can really hammer the chlorine levels without risking damage to the pool. If it's vinyl, it's a balancing act, depending on the level of stabilizer. Look for our Best Guess table for levels of chlorine re: stabilizer.