Sagina?

Here is a link that might be useful: {{gwi:289993}}

I think saltcedar got it. The plant looks like one I've been hoping to see here - maybe Sagina procumbens (Matted Pearlwort). That one has just 4 petals, instead of the usual 5, and seems to match your photo.

I also have some volunteer non-Irish moss which impresses me as more "weedy" (less thick or compact) than the "true" Irish moss I've seen over the years. My volunteer has the nasty habit of dying once it's set seed, and I'm unable to find any at the moment in its usual location to count the petals and sepals.

Here's USDA's page for the various Saginas:
http://plants.usda.gov/java/profile?symbol=SAGIN
Five are listed for Washington state, as well as two others for British Columbia.

Years ago when I was trying to identify mine, I never got as far as figuring out whether it was S. decumbrens or procumbrens. But I did save a link to this page with good photos of the 4-petalled S. decumbrens:
http://oregonstate.edu/dept/nursery-weeds/weedspeciespage/pearlwort/pearlwort_page.html

Thank you, saltcedar, jekeesl, and missingtheobvious! This was about to be my second "Oh, that would look great in my garden so I'll spread it around" mistake (the first was foxglove). It's too bad, though, because it looks attractive and slugs don't touch it at all compared to my now slug-decimated true mosses like Irish and Scottish. Time to adding weeding to my to-do list. Thank you again for your selfless aid!

Ummm, Irish and Scotch moss are not "true" mosses - they are just another species of Sagina (Sagina subulata) like the photo above, both of which seem to be quite slug resistant in my garden. They do need decent, well-draining soil to thrive and are not the best choice for a lot of shade.

Creeping thyme would be a reasonable substitute for your Irish or Scotch moss if slugs really are an issue.

Slugs, a shady spot, and caterpillars were too much for my Irish Moss. Thank you for the "true mosses" correction. Creeping thyme looks promising. Thank you!