Revision - Not a Hybrid Butterflyfish
Being a coral specialist, (although learning about fish as fast as I can) I thought this butterflyfish seemed to be a hybrid between a Spotfin Butterflyfish and a Foureyed Butterflyfish, and my quick search for information did not help.
It was found by Jessie Hallett today as we were setting up an experiment out in the lagoon.
Judie Clee - a local REEF expert on fish identification, was able to correct our mistake:
Turns out the dark spot is part of the nocturnal marking of the fish - and it will get much darker at night, while the little black spot near the tail will fade. Perhaps this guy was getting ready for night at 3pm, since the days are so short right now.
You can read more about the species (Chaetodon ocellatus) on FishBase: link.