Neon Damsel or Blue and Gold Damselfish (Pomacentrus coelestis) in the Coral Gardens between two Motu on the Tahaa North Reef, about 2km off the coast of Taha'a, Leeward Islands, French Polynesia. Photographed on 29 July 2017.

fishbase.tw/Summary/SpeciesSummary.php?ID=5720

The left-side individual, with a Cassius Blue from the LRGV for comparison.

I have a theory about IDing this species from Cassius Blue:

Note the arrangement of dark marks underlined in pink:

For Marine, the dark block in the fourth inter-veinal wedge (the wedge labeled with a green "4") is usually in-line with the outer three blocks; this individual Marine has a slight "uptick" in the 3rd inter-veinal block just before it meets the 4th, but the 4th inter-veinal block is still mostly in-line with the others.

For Cassius, the 4th inter-veinal block is sharply offset basally such that its upper/outer corner touches (or is separated from) the inner/lower corner of the 3rd inter-venial block.

I have been checking this feature for many years, and I feel that it is a very reliable means of distinguishing between the two taxa.

Size comparison with creeping phlox (Phlox sp.) flower. Legs very red, not pale (thysbe) or black (diffinis). No good wing shots, but second image shows shadow of wing with smooth edge to red marginal band.

About 12 cm long, laterally compressed, in ~10 cm water depth on sandflat at low tide (hence horrible photos), acting like a leaf. White patches on belly may be missing scales. Very unusual fish. Labridae has been suggested but I can't find anything resembling it.

PHOTO CREDIT:
Photo 1: Joe Platko https://www.instagram.com/hiimjoe88
Photo 2: JR Sosky https://www.instagram.com/hijrsosky/
Photo 3: JR Sosky/Marianne Nyeggard

MOLA TECTA IN MONTEREY BAY! Photographed by local photographers Joe Platko and JR Sosky, and Identified by Marianne Nyeggard via Michael Howard at Monterey Bay Aquarium