Too late to prevent it...How do I kill algae?
First off, balance your water, paying particular attention to pH. Secondly, check your filter system and clean if necessary. Adjust valving for optimum circulation and allow it to run 24 hours a day until the pool clears. Turn on automatic cleaners to help stir things up. Backwash as necessary.
For suspended green algae, shock the pool...hard. Put in as much hypochlorite as it takes to turn the pool a cloudy, bluish/gray color. Brush the walls and floors towards the main drain. Backwash the filter when the pressure gauge indicates the need (8 - 10 lbs. above clean reading, after backwashing.) Using a flocculent may be a good choice if the pool is extremely "swampy". If you cannot see the bottom of the pool, and it is filled with leaves and debris, it may be wise to drain the pool, acid wash and refill it.
After the chlorine level has come down below 5 ppm, add an algaecide and brush the pool again. When it all settles, vacuum the pool (to waste, if possible). Check and re-balance the pool water if necessary.
For algae which is not suspended, but only clinging to the walls, follow the same advice above, first shock with brushing, then add an algaecide, brush again, vacuum to waste (preferred) or vacuum and then backwash the filter. Use of a steel bristled brush is recommended for algae on plaster pools (use nylon brush on vinyl). Filter, Filter, Filter!
For black algae, the brushing part is very important. You must tear through the protective layers so the chemicals can destroy the plant from the inside out. Pumice stones work well to knock off the heads of black algae. (Don't forget to vacuum them up later, and backwash them out of the filter ASAP). Also effective on the black algae nodules is sprinkling granular trichlor over the spots (of course if they're on the wall this is next to impossible). Rubbing the spots on the walls with a trichlor tablet or stick can also be effective to knock off the heads and get trichlor directly to the roots. Follow up with a dose of copper algaecide, or high strength polymers. Simazine, an herbicide, was a very effective black algae treatment, but is no longer available in America.
If algae has been an ongoing problem in your pool for several years, you may do well to drain the pool. Many years of algae builds up dead algae cells and lots of other solids in the water that contribute to its rejuvenation. Acid washing and/or pressure washing is preferable once drained, to kill the roots of the algae embedded in the plaster. NEXT: Change the sand if you have a sand filter or change the cartridge if it is a cartridge type. Sand should be changed every 5 - 7 yrs (or every 2 if you use Baquacil), and cartridge filters should have new elements every 1 - 2 years. If you have a D.E. filter (good for you!), you should remove the elements, spray clean, soak in a 10 : 1 water/ bleach solution, rinse and replace. A well functioning filter will prevent algae.
Another item to look at is the method of sanitation and the type of filtering you have. Far too many pools out there were sold with marginal filter systems, meant to run 24 hrs per day. Well, these systems get old and tired, or the new owners only run it 12 hrs per day (or less). For good algae prevention, we need a combination of good filtration, sanitation and circulation. It may be time to consider changing the old pump and filter. It's cheaper and easier to pay a little up front for more chemicals, electricity or better equipment than all the money and aggravation spent on fighting algae blooms.