Primary phase colouration is a dull gray with scattered white spots, gradually becoming uniformly dark green.

This species is gregarious and usually occurs in small aggregations, but group size can be quite large on seaward and clear outer lagoon reefs, exceeding 75 individuals.

The green humphead parrotfish, is sexually monochromatic, i.e. there is no initial or terminal phase in the life cycle of the adults.

Newly settled juveniles are found in branching coral habitats (primarily Acropora) in sheltered lagoons.

Juvenile habitats are susceptible to being degraded by poor water quality, such as run-off of sediments from logging.

[14] The waters surrounding Wake Island, Johnston Atoll, and Palmyra Atoll from the shoreline out to 50 fathoms (91 m) are protected as low-use marine protected areas, which means any person of the United States fishing for, taking, or retaining this fish must have a special permit.