I had been calling these Protaeolidiella atra, following the Sea Slug Forum, but they are listed as Pleurolidia juliae on Sea Slugs of Hawaii. If they are different species, the difference appears to be the presence of a dorsal white stripe on P. juliae, while the body of P. atra is all black. The animals are very hard to see on their prey hydroid, Solanderia. The easiest way to spot them is to look over every colony of Solanderia you see until you spot the egg masses deposited in an orange ribbon spiraling around branches of the hydroid. If you see those, look close and a nudibranch will likely be present. I saw several specimens off Koko Head, near the Halona Point Blowhole.
The nudibranch is on the upper right branch of the photo below.
Created 20 April 2009
Updated 11 August 2013