In my defense this was a relatively small slug. Well, small by SCUBA standards, so maybe 2 cm, which is quite large by CA tidepooling standards.
I think this tern looks like a drawing of a bird by someone who doesn't know how to draw very well. Other than that, I was very excited to see a new bird during my last couple hours in Hawaii. Not sure why I didn't see it somewhere else, as this pair of white terns were 10 feet above a very busy beach.