What you’re seeing is the defense mechanism of a tiny crustacean called an ostracod, a shrimp-like organism about 1mm in size that some fish accidentally eat while hunting for plankton. When eaten by a translucent cardinalfish, the ostracod immediately releases a bioluminescent chemical in an attempt to illuminate the fish from the inside, making it immediately identifiable to predators.
Copied this from Reef Central, thought it would be awesome to share.