"It's a stout one and on the large end of the size range, but your crab is almost certainly a northern kelp crab (Pugettia producta). Perfectly native and natural. It clings to and feeds partly on kelp and is also often found on pilings. The one you photographed looks like a dead one, but often you'll find molts washed up on shore (shed their exoskeleton regularly). The only way to know for sure would be to try to lift up the back of the carapace (top sheriff badge looking part) with a finger nail. On a molt it usually comes right up to reveal a relatively clean interior. "
