This clue would have introduced you to the world of biological luminescence or bioluminescence.
This biological phenomenon can bee seen in around 1,500 organisms, especially in many marine organisms.
Dinoflagellates typically produce a blue-green light due to cytoplasmic bodies called scintillons which can produce the biological reactions for bioluminescence. The luminescence of this species is usually triggered by mechanical disturbances such as waves they will emit a brief blue flash.
The way the scintillons operate is by having maximum amounts at night breaking down during the night, so the bioluminescence can only be observed at night. These dinoflagellates live in fragile ecosystems that are extremely rare and use their bioluminescence as a defense mechanism. By lighting up in small flashes, any marine being that tries to consume the dinoflagellate will have its position compromised, so predators higher up the food chain can easily find them.
For more information on dinoflagellates as a whole species you can begin at the Wikipedia page: https://en.wikipedia.org/wiki/Dinoflagellate#Bioluminescence.