Split off of this post. JasonLion
I don't know if this is similar to what I ran into, but I kept getting 'stuff' settling in the dips/crevices of my liner that looked similar to your photos, but the overnight FC loss test never dropped more than 0.5 and CCs were usually 0.0, sometimes 0.5 (color change for CC test was very slightly pink sometimes). The pool eventually starting getting cloudy. I tried shocking; that cleared it up, then a few days later, the sediment was back.
I tried cleaning inside my steps & finally just pulled them out -- they dumped green water into the pool & the step weight's water was even greener. Circulation in the steps & weight must have been pretty poor, because earlier shocking obviously clear the stuff out of them. After shocking again, the pool has stayed clear w/nothing but obvious dirt & leaves settling to the bottom.
I'm fairly new to this, but it seems like there was algae in the steps & weight that didn't register much of anything on the overnight FC loss & CC tests, but which kept adding dead algae or whatever to the bottom of the pool. If that seems like a real explanation to the experts here, do you have anything in the pool where algae could be hidden & growing?