This is a White-crusted Cuthona (Cuthona albocrusta) among some hydroids. These nudibranchs are small — this one was probably just under 1 cm long. [And yes, that's another even smaller nudibranch above the Cuthona. It was so tiny that I didn't even see it when I took this photograph. I think it's a species of Doto.] Here's another view:
I wasn't sure if my field photographs would come out, and since I hadn't documented this species for Bodega Head yet, I brought it in for a few microscope images.
You can see why it's called "white-crusted," although I think "frosted" would be another good description.