Help identifying mollusk - Oxnard, CA, USA

I wanted help identifying this mollusk. It looks kind of like an oyster. I have been coming to Channel Islands Harbor since the 70’s. My brother and I played on the rocks, beaches and docks. These started appearing around 10 years ago, after the starfish died off.

My guess it is either an invasive species or starting thriving after the starfish started dying off.

Picture taken in Channel Islands Harbor in Oxnard, CA.

Any thoughts?