What are these stains/blobs in my pool??

The deep end of my in ground pool has brown stain area that does not respond to brushing. These little blobs don't either. We redid the whole bottom of the pool about 4 years ago. Yesterday I went in with goggles and a scrub brush and they do not budge. I even tried sandpaper. Any idea what these could be? We recently had algae and I thought it was related but now I don't know...
 

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Shocktop2000

Member
Jul 16, 2019
13
Houston, Texas
You ever get this figured out. Mine is not as bad as yours, but those stains look very similar to the ones growing in my pool. Shock, Jacks Magic, and Vitamin C do not budge the stains
 
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