Do you have a pool vacuum? You could run it for a couple hours when you can visibly see the stuff on the bottom and it should suck it up and remove it from your pool. If it comes back then, its algae. If it doesn't come back, probably dirt/clay. I had what I thought was algae in the creases of my liner when I opened this year, except every test pointed to no algae and the water itself looked perfect. Turned out it was just dirt and clay that looked a little brown.
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Also, can you add your pool stats like equipment, size, accessories, etc to your signature?