What it is & what it looks like:
a naturally occurring fungus (this is NOT a form of Algae; it is animal not vegetable)
White mucous or tissue paper-like substance (sometimes called "toilet paper syndrome").

White Water Mold forms a heavy, protective coating providing the organism with an unusually high level of protection that is very resilient against chlorine, bromine & non-halogen sanitizers or germicides - in other words, shocking (even in large amounts) will have little effect.

White Water Mold can remain a contaminant even after treatment. It exists on the surface of PVC plastics & can re-contaminate long after it looks like it has been destroyed (includes pool & spa toys, floats, spa pillows, steps, hydro-jet rings, filter areas, skimmer baskets, weirs, garden hoses, lights - as shown left, etc.). Small quantities can lead to a re-establishment of the problem caused by improper water & spa maintenance, environment.

White Water Mold likes areas of poor circulation & prefers areas that are "dark" (not exposed to direct sunlight) & with "slow moving" water. It is often found alongside of "Pink Slime".

Spa mold gets a hold on your spa due to the growth & spread of bio-films on spa surfaces. Get the details here.

White Water Mold IS NOT CAUSED BY USING BIGUANIDES (BaquaSpa, Soft Soak, Leisure Time Free, 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 Spa & Hot tub surfaces weekly, including steps (especially vertical surfaces each step) & rails
2. Expose ALL pool surfaces to as much sunlight as possible (sunlight & UV are natural
oxidizers -  often not practical on indoor spas)
3. Remove the lid from the skimmer to allow sunlight into the basket for several hours each day ** INGROUND SPAS 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 HELP 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 - White Water Mold & Pink Slime is typically introduced to the spa or hot tub from the garden hose.
6. Regularly clean toys & floats
7. Regularly clean spa solar or floating blanket - Natural Chemistry works well 
8. Chemically clean filter cartridge every 4 to 6 weeks 
9. Add regular maintenance dosages of "Shock" every week as prescribed
10. Use SpaBoss Ultra Conditioner as a preventative measure
11. Run the filter a minimum of 12 hours daily to prevent "dead spots" in the spa
12. Remember to clean & rinse the brushes, hoses & vacuums that you use to clean the spa
13. Leave as much of your spa 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 affected all spa & hot tub 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 SpaBoss Ultra Conditioner levels
9. Maintain good water balance of pH, Total Alkalinity & Calcium Hardness
10. Be sure to use Spa System Flush, Spa Purge, AquaFinesse SpaClean or Swirl Away whenever you drain & refill the spa to remove accumulations of bio-film from the plumbing lines.