This is a palenose parrotfish passing through a school of whitebar surgeonfish. Most parrotfish attain this kind of color when they reach adulthood. Often times it appears somewhat muted, but when the sun catches them they are iridescent and stand out as much or more than anything else in the water.
In my attempts to identify what I see in the water, I use John P. Hoover’s book The Ultimate Guide to Hawaiian Reef Fishes, Sea Turtles, Dolphins, Whales, and Seals. His website is hawaiisfishes.com.