Algae

• Algae are basically extremely tiny plants. They're microscopic in size, in fact, and aren't noticed by pool owners until there are a great many of them present in the water. Almost all algae use photosynthesis to produce energy, much like most other plants. Combine that ability with water, and the potential for havoc in a pool can be relentless. Like all others, mustard algae finds its way into a pool via the wind, rain or even on the bodies of swimmers.
  
  

Conditions

• In a pool, conditions have to be correct for mustard algae to gain a foothold in its water. For one, disinfectant levels have to be inadequate. For most pools, that means not enough chlorine. Recommended levels should be at about 2 parts per million PPM for optimum effectiveness. When there's little chlorine present -- below 1 PPM, for example -- algae can thrive. Also, a general lack of proper pool chemical balance can be a contributor to the problem.
  
  

Mustard Algae

• Like every other algae, the mustard yellow kind can literally bloom overnight if a pool's not sanitary enough. Once in the water and soaking up a lot of sunlight and warmer pool temperatures, they can multiply extremely fast. The best preventative, of course, is proper chlorine and pH balance. However, once a pool's contaminated it'll come down to working diligently to eliminate it. That usually means chemical treatment and scrubbing of pool walls.
  
  

Elimination

• Unfortunately, mustard algae is one of the toughest algae to eliminate. Superchlorination, or "shocking," is going to be needed, at minimum. That involves raising pool chlorine levels to about 10 times the recommended 2 PPM and then letting the pool sit for several hours. You'll also need to make sure your pH is within the 7 to 8 range -- 7.4 to 7.6 is best. Lastly, you'll need to scrub walls, vacuum and superchlorinate again if necessary.
  
  

Algaecides

• Because mustard algae is so hardy, you'll also have to employ a strong algaecide formulated to work against mustard algae. A copper-based algaecide can also work well. Use it after shocking and scrubbing of pool walls and other surfaces has occurred. It's a good idea to apply algaecide in the morning so that it can work throughout the daylight hours -- when algae is most active just like any other plant.