Plaster or rocks??

Looks like chunks of plaster from a slob getting some into open pipes during construction. You're very lucky they came loose and came through without clogging a pipe somewhere underground. If any of your returns look weak, you know what to look for now. People are always finding bits of stray plaster behind the eyeballs.
 
I built 2 pools by 2 different pool builders in the last 5 years. In the neighborhood of 1 year in, I found similar things in my filter baskets. I never could find out where they came from, especially in my current pool where the plaster is blue and the "rocks" were white.
 
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