Parrotfishes resemble wrasses, but differ in their beak-like and strong teeth. All are herbivores. Most of them scrape thread algae from dead coral rocks. They also feed on leaf algae, seaweed and living corals. Their excretions make them the largest producers of lime sand on the reef.
They sleep at night in a self-produced "slime cocoon". They change colour with age and sex. Mostly they mature to the grey to brown initial phase (IP) as females and occasionally as primary males. Some females turn into colourful green to blue super males. Teminal Phase (TP)

Excerpt from "Life in the Red Sea." iBooks.