Just wanted to add some experience. I recently completely a slam with success given a green algae outbreak. But, I've struggled from time to time with the "dirt" issue that I've thought was mustard algae. Not sure in the past what it was. But, after I slammed, I was still getting areas of the pool with the dirt, very similar to the last pic posted, but not as much. It develops in the same spots on the floor. I had my pool up to mustard level algae for 24 hours after the slam, but the "dirt" kept coming back. A few years back I was having a similar issue and also a clouded pool caused by residue coming off my fiberglass pool. At that time, it was recommended I use a Slimebag. It worked very well. I hadn't used the bag in a few years, but I just used it to see if I can address the current "dirt" issue. I'm still experimenting. But, when I attach the bag to one of my returns (i just have two returns) the problem does NOT appear. When I do this, the return the bag isn't attached to has significant less pressure. The pool clarity is amazing. When I remove the bag, both returns have great pressure, but then I'm getting the "dirt" back after 12 hours or so. Given the fact the bag, when in use in only one return, eliminates the problem, I'm pretty sure it's not mustard algae. I'm going to try to use the bag on the other jet to see what happens. But, I'm sharing this in the hopes that perhaps additional filtration help could solve the problem if it ends up not being algae. I know many add DE to a sand filter to help with filtration. I've not done that in a couple of of year, but if I may start doing this again if the bag, with its additional filtration capabilities, can eliminate the "dirt" issue.
Good advice above to clean out light niches. They tend to get very gross.
