Half an hour later, this critter was out exploring ...
It's a Spotted Aglaja*. It was probably buried in the sand, where these usually hang out, just under the surface.
... it preys on bubble snails along with other small mud dwellers including copepods. Lacking a radula or rasping tongue, it sucks in it's prey whole. (From Puget Sound Sea Life.)Here are a few more photos.
These animals are hermaphrodites; each one is both male and female. There's a good series of mating photos of a similar species on DORIS. (In French, but Google will translate, sort of.)
Mine (Spotty) is a young one; the average size is about 3/4 of an inch, although they grow up to about 2 inches.
After s/he did the rounds of the walls, s/he headed down towards the sand, and hasn't been seen since. There's plenty there for her to eat; copepods, amphipods, a few tiny clams, and loads of infant polychaete worms.
* Or possibly Diomede's Aglaja, but I think the head on DA is too rounded and narrow.