Could this be the pincer of a Pagurus samuelis or Blueband hermit crab? It was a live pincer but inside a hole in a rock, not inside a shell. We saw it moving. The pincer has beautiful bright blue untidy spots, and it was less than an inch long. We couldn't see the rest of the crab.
In shells of Olivella biplicata
Pagurus samuelis, the blueband hermit crab, is a species of hermit crab from the west coast of North America, and the most common hermit crab in California. It is a small species, with distinctive blue bands on its legs. It prefers to live in the shell of the black turban snail, and is a nocturnal scavenger of algae and carrion.