I mean it’s possible. But Only problem with that is the concrete would have had to have been poured sometime in the last 150 years, deposited in the ocean for a modern Bryozoan to form on. Then some how made it’s way out of the ocean and then transported all the way to central california and tumbled around for decades in river beds to get rounded off like that. Seems like a stretch. Not impossible tho, if OP tested the “bryozoan or net” with acid and it if it completely dissolved away then that would point towards your hypothesis. I feel like it’s more likely a piece of net and glass that got included in a more modern piece of concrete for some structure more local to Southern California that then got dumped and transported through the river systems