Here's the Rock Sandpiper (lower bird) with a Surfbird for comparison:
And one more, this time with a Surfbird (left), a Black Turnstone (right), and the Rock Sandpiper (bottom):
Today I almost would have called it a "Seaweed Sandpiper." Although rock was the base material, the sandpiper spent most of its time feeding very intensively among the seaweeds (primarily Pyropia sp., formerly Porphyra sp.) high on the rocks. I couldn't see what the sandpiper was eating, but perhaps it was finding amphipods?
P.S. If you're interested in seeing this bird, it's been pretty easy to observe from the main whale watching area at the outer parking lot on Bodega Head. Check the large sea stacks just offshore from the whale watching area (e.g., when you're at the whale watching area, look down and a bit to the north).