I find it nearly impossible to tell the difference between the Gymnodoris bicolor and Gymnodoris sp.#2.  According to Sea Slugs of Hawaii, the primary differences are that Gymnodoris sp.#2 has more pronounced nipples on the tips of their rhinophores (often tinted orange), and their genital pore is forward of a more closed horseshoe-shaped branchial plume rather than behind a more open horse-shoe shaped branchia.  So pay attention to the right side when photographing — look for a small dent or hole in the nudibranch’s skin and you may be fortunate enough to distinguish between the two.  I believe I have this right, but who knows. I will happily stand corrected!