Paracheilinus mccoskeri.
I added 'wrasse' to the official name, because it is wrasse rather than sex offender. The flash comes from the first soft ray of the dorsal fin (which is roughly half way along the fin) which grows much longer then the other spines or rays, forming a long coloured filament which can be raised vertically. Unfortunately this fish only had a very short one and as it usually swam with its dorsal folded (as shown) it was rarely visible at all.
