At the time we referred to you as “Dude,” and the additional members of your species we subsequently came across we referred to as “more Dudes.”
I took a few pictures of you through the water using my old camera, Mr. Hesitation. Your lack of edged-ness certainly didn't help with the focusing, not to mention your colouring – beige on beige? You aren’t exactly a Spanish Shawl, are you? In fact, you’re not a nudibranch at all.
Bear in mind that I am busy, have things to do, obligations, the crocodile pool needs reinflating. So I can only afford 15 minutes to identify you. If I misclassify you, or can only accurately get down to say, Family or Genus, well, so be it. Also consider that many mudflat adventurers, upon seeing you, would consider you a small, mysterious blob, and have forgotten all about you by the time the tide had chased them back to shore.
Here we go. After fifteen minutes of googling, looking back and forth at images – from British Columbia, California, Puget Sound – I believe you to be an Opisthobranch Mollusk of the suborder Cephalaspidea, that is, a Headshield Slug. Sorry it’s not a pretty name, like Spanish Shawl, but then again, you aren’t a nudibranch (in the suborder Nudibranchia).
I strongly suspect you to be in the Family Aglajidae, which sounds like someone trying to speak with a mouth full of hot lasagna. You may even be in the genus Melanochlamys, but I wouldn’t put money on that. Species? M. diomedea? I can’t say. Sorry, your 15 minutes is up, Dude.