What do you apply to clean algae off plaster pool walls?

First, you need to clean the tiles with a product that will remove soap scum. Scrub or brush both the tile and its grout. Then spray a bleach solution (1cup of bleach to 1 gallon of hot water) and let sit awhile. Do this only in well ventilated areas. Wear gloves, and old clothes you don't care about in case the bleach splashes. Depending on how long since the grout has been cleaned, you may need to repeat the process several times. If you notice the grout is cracked, or missing you will need to scrape it out (an old screwdriver works well) and replace the grout when the whole area is dry.