I'm currently in the process of shocking my pool for mustard algae. I've scrubbed just about all the surfaces, but there is an area I'm not sure about. It is around the main drain, in the deep end of the pool. There is a large area of brown stains that I can't scrub away, and they don't look that much different than the mustard algae on the side of the pool. However, the main drain gets a fair amount of sun, so I'm thinking they really might be stains. The only way I can get to the spots is with the pool brush because it is in the deepest part of the pool. Using the pool brush, nothing brushes away in that area. However, it is also the farthest spot I have to reach with the brush, and I may not be able to give a scrub good enough to dislodge any algae. I'm posting two pictures of the spots, though they aren't that great. It is breezy today, so the rippling water surface impedes the view.
Should I be concerned?