Maybe this:
Banana Slug (Ariolimax columbianus

After I saw your post and searched I'm thinking Albino Ghost Slug, or Possibly just an Albino Slug? your Banana Slug hypothesis led me to this. The top head texture is just so scaled like human or lizard skin, but then each slug is different in genetics right, thus how via evolution we have different species, different genetics and radiated or otherwise mutation. Are albino slugs rare to Washington state forests?

The Great Gray slug is actually spotted and this one isn't yellow enough to be a banana slug or white enough to be an albino slug. I think it is probably just a run-of-the-mill garden slug.

they can be brown too

Here is a link that might be useful: Wiki

Still doesn't look like a banana slug.

There are a few to choose from, this being the slug capitol of the world :)

Here is a link that might be useful: WA slug ID help