I have only ever seen one of these Elysia obtusa– a juvenile that was no bigger than 4 mm. Its bright emerald green colour threw me off an initial identification as these are usually more of a goldish-yellow colour.   There is some suggestion that the strong green colour corresponds with a recent feeding and even in this tiny animal the green digestive glands are quite obvious.  Found in less than 1.5 metres of water in a semi-protected surge zone in North Kihei, Maui.  For more information and photos see Sea Slug Forum and Sea Slugs of Hawaii.