Mustard algae is a particularly persistent kind of algae, that appears to go away when you SLAM the pool normally, only to reappear as soon as you come back down to normal chlorine levels. It tends to grow in clumps on surfaces at normal chlorine levels, and varies from a true “mustard” yellow to green with hints of yellow. It can also grow free floating in the water if the chlorine level is low enough.
Most people who think they have mustard algae don’t actually have mustard algae. Pollen is often mistaken for mustard algae, since it often collects in very similar looking patches. Mustard algae tends to grow on the walls and floor of the pool on the shaded side. Pollen can look very much like mustard algae, but it will only settle on the bottom of the pool, not the walls, and appears everywhere, not just on the shaded side. Another way to distinguish between the two is that when you go up to shock level and brush the pool, mustard algae won’t come back as long as you stay at shock level. Pollen continues to accumulate, even at shock level.
To permanently get rid of mustard algae, you first SLAM the pool normally, following the directions in How to SLAM Your Pool. Once you have completed SLAMing your pool in the standard way, raise the FC level up to the extra high mustard algae shock level for 24 hours. Use PoolMath, or see the Yel/MstrdShock column in the chart in this post, to find your mustard algae shock level.
While at mustard algae shock level you need to throughly brush the entire pool, including walls, floor, steps, skimmer, under ladders, everywhere. All pool tools and toys and anything else that is used in the pool needs to be wiped down with a dilute bleach solution or soaked in the pool for an hour while at mustard algae shock level. Bathing suits should be run through the laundry. If you have underwater lights, they should be removed from their niche and the niche brushed out, and then the lights replaced. If you have fountains, or waterfalls, or other water features, they need to be run for at least an hour while at mustard algae shock level. Be very thorough, or you will most likely just need to do it all over again.