Blue flash

Mazzaella splendens, Iridescent (or rainbow) seaweed. This is the one that causes the "blue flash" you might see from the bluff top on a sunny day, looking down into the tide pools.

 

This one is at Moonstone beach in Cambria, just south of the surfers point-break. Quite a nice selection of these guys in the tidepools there.

 

Here's a bit more info, from a good Stanford website:

seanet.stanford.edu/red#splendens

Done