It’s a liner gone extremely wrong. Makes sense now, home done liner they tried to get it all along the tank failed then left tank sitting upside down on table or bucket with too much excess liner within tank. maybe they didn’t follow manufacturer instructions for curing liner conditions so it all settled at deepest point of tank while upside down, Hardened up and bam. Looks like they didn’t properly clean tank when they lined it as well bunch of particles within liner. Gotta figure out how to remove liner. So far acetone does absolutely nothing to it. Really hope I don’t have to use crazy strong acid to clean it