This is a pair of Japanese blacktail triplefins (Spingerichthys bapturus), known as hime-ginpo (ヒメギンポ) locally. The one in front is the female, the orange one to the rear the male.
Apparently, they’re normally relatively drab, uninteresting fish. The individuals pictured here are dressed to impress. More specifically, these are their mating colours.
Scampering across the face of a vertical slab of rock, this pair appeared to have decided on one another already, and probably mated in the evening.
As a prelude to mating, the female stopped every once in a while and wiggled her tail in an enticing manner, which inevitably sent the male into a flurry of frenzied activity…darting to and fro…eventually coming full circle and ending up next to his companion again, who maintained an air of innocence throughout.