Cohen and Carleton refer to this as the Lake Merritt Cuthona, which is cool but slightly misleading, since it is thought to be introduced, even though its native range is unknown. We found a couple of them among the mussels and anemones on the shadier sides of the docks by the Lake Merritt boathouse. This one was 7-10 mm. Many thanks to Robin for leading me on the path to finding these guys.