Who's hiding in this tide pool? It's a green crab, half-hidden under a rock.
Despite their name, green crabs can vary in color from green to brown to reddish-orange. The crab here has a mottled appearance which allows it to blend in nicely with the surrounding rocks.
Green crabs are a common find in tide pools along the East coast of the United States, but they are not native to the area. The green crab originated in Europe, but was introduced into the U.S. in the 1800's, possibly from the ballast water in ships.