This is a fish of many shapes and colors. Seen from the side they look skinny and long. From above they look much wider and, consequently, seem shorter. They look silvery from the side, green from above and they’ll add dark bars sometimes.
This one was hanging out in shallow water and showed no inclination to move on. It might have been guarding eggs, but that’s just speculation on my part. It shows a green tint and also the blue spots and lines that give it its name.
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.