I saw a couple of these little guys over memorial day weekend in the shallows at Madrona Wall on Vancouver Island. I am thinking it is some sort of very juvenile gunnel or prickleback, but I really have no idea. It was about the size of a dime and really colorful.
It is described on page 69 of Beneath Pacific Tides by Greg Jensen and on page 120 of Marine Life of the Pacific Northwest by Andy Lamb (under an older name).