|The family Chromodorididae
includes species that are often brightly colored and
elaborately patterned, presumably advertising their distastefulness.
Most chromodorids store toxic chemicals from their sponge food in
mantle glands, usually
concentrated in a submarginal band on the notum, in order to repel
predators. The most brightly colored species are diurnal but some of the others are nocturnal. It
is the largest Indo-Pacific nudibranch family, with more than 600
species, and it is well-represented in Hawaii with about 45 species
in 11 genera (Ardeadoris, Ceratosoma, Chromodoris, Diversidoris, Doriprismatica, Glossodoris, Goniobranchus, Hypselodoris, Mexichromis, Thorunna, and Verconia ).
There's some chance that this photo by Chen Zhi shows a Ceratosoma gracillimum but there's insufficient detail for a firm ID.