White Water Mold (WWM) is a naturally occurring fungus (this is NOT a form of Algae; it is animal not vegetable) that resembles white mucous or tissue paper found all over pool surfaces. 
forms a heavy, protective coating providing the organism with an unusually high level of protection that is very resilient against halogen-based (chlorine, bromine) & non-halogen sanitizers or germicides

White Water Mold is a biofilm.  Here's more about biofilm.

can remain a contaminant even after treatment

exists on the surface of PVC plastics

can re-contaminate long after it appears that it has been destroyed (includes pool toys, floats, ladders, steps, fountains, automatic pool cleaner parts, skimmer baskets, weirs, directional fittings, garden hoses, etc.)

small quantities can lead to a re-establishment of the problem

caused by improper water & pool maintenance, environment, poor circulation

prefers areas that are "dark" (not exposed to direct sunlight) & with "slow moving" water

White Water Mold IS NOT CAUSED BY USING BIGUANIDES (Soft Swim, Baquacil, Polyclear, etc.)! It is an environmental issue.

Prevention:
Prevention of "white water mold" is preferred over treatment. Follow these steps to help prevent white water mold:
1. Physically brush & clean ALL Pool surfaces weekly, including ladder steps (especially underneath each step) & rails
2. Expose ALL pool surfaces to as much sunlight as possible (sunlight & UV are natural
oxidizers)
3. Remove the lid from the skimmer to allow sunlight into the basket for several hours each
day ** INGROUND POOLS MUST USE EXTREME CAUTION in doing this in order to avoid
a person falling into or otherwise injuring themselves due to an open skimmer.
4. Regularly add oxidizing chemicals into the skimmer to purge & clean the filtration lines of
any bio-film*
5. When adding make-up water from the garden hose, allow the water to run for 2 to 3
minutes before putting the hose into the pool.
6. Regularly clean pool toys & floats (use Stow Away)
7. Regularly clean pool solar blanket (use Stow Away)
8. Chemically clean pool filter every 4 to 6 weeks (use Strip Kwik, Kleen It or SoftSwim® Filter Cleaner)
9. Add regular Maintenance dosages of "Shock" & Algicide every 2 to 3 weeks as prescribed
10. Use Optimizer Plus® as a preventative measure
11. Run the filter a minimum of 12 hours daily to prevent "dead spots" in the pool
12. Remember to clean & rinse the brushes, hoses & vacuums that you use to clean the pool 13. Leave as much of your pool equipment exposed to the sun
14. Keep the water balanced at all times. Recheck after heavy usage or rain or large "top-offs" of new water. Water balance refers to Free Available Sanitizer level, pH, Total Alkalinity, Calcium Hardness.

Treatment:
Treatment of "white water mold" MUST BE QUICK AND TOTAL! DON'T SKIMP!
1. Clean all pool & affected surfaces as prescribed above.
2. Physically clean & remove all visible "white water mold"
3. Add an initial dosage of algicide to the pool*
4. "Shock" the pool with a triple or quadruple dose*
5. Run filter 24 hours daily until water is clear & halogen or peroxide levels are maintained at a "higher" level*
6. Chemically clean the filter
7. Bring in a fresh water sample about 4 days later for a retest & evaluation
8. Maintain Optimizer Plus® levels
9. Maintain good water balance of pH, Total Alkalinity & Calcium Hardness

To "cure" the problem, take a look at AquaFinesse Pool Pucks.

Download FREE brochures to learn about pool care needs & problems.

*Follow the instructions given by your pool care specialist EXPLICITLY. Failure to do so could result in harm to you and/or your pool & its components.