That is dead algae and some dirt from the air. The reason it collects in the same spot is that is the lowest spot(s) in the pool. Once the water is still it settles out of the water and ends up there. The down side of a sand filter is the fact it does not always catch the finer stuff in the water

Until you can "catch" all of the stuff and get it up and out of the pool it will settle out of the water waiting for your filter to catch it or you vacuuming it out of the pool by waste (which you have been trying to do with not much luck

).
Have you added any DE to your sand?
It seems to be the opposite, or I might just be reading incorrectly. The dirt and algea settles in that spot, but only when the filter is running. I had a crack turn into a massive leak last night in the hose from the the skimmer basket to the pump. I noticed it at about 21:00, and there wasn't anything open. With COVID even walmart closed at 20:30. So my pump did not run last night until about 0815 this this morning.
My pump has a valve, so I closed that, disconnected the line from the pump and duct tapped it over the edge of the pool (cut line was right near the inlet to the pump) so it wouldn't leak all night. It was a split about 3 / 4 the way around, so it was a lot.
I did clean the algea from the bottom before I went to bed. When I woke up this morrning, NO ALGEA/DIRT in the center. If algae was growing or multiplying, wouldn't it come back without running the filter?! Seems like the algea is dead!? But, when I collect it, it does still poof!
Is that normal for dead algea? I don't know, I have to ask.
On that note, when I turned the filter back on, less than an hour later, there was accumulation in the middle of the pool again.
I have not added any DE to the sand fitler. Do you recommend this? How much do you recommend I add? I don't know anything about DE, I know they sell DE Filter Aid, is this the same? I've read about adding a little bit to a sand filter to increase filter capability, and I'm happy to do so.
Yeah, other than that small settling area the water looks great.

Have you done an
Overnight Chlorine Loss Test lately? I'd be surprised if you didn't pass.
Since I dropped the 2 gallons into the pool, I haven't done another OCLT. But I have passed 2. I'll try and do another one tonight. I might have enough reagent left, especially seeing as though we've had 2 very sunny days, my levels should have some down significantly.