The small water born plants or animals are known as plankton. They can be either phytoplankton or zooplankton.
The relationship between the the distribution of coral polyps and the population density of plankton is that coral polyp depends on the plankton for their nutrition.
Most of the coral polyps like zoanthids depends on phytoplankton and zooplankton for their food(nutritional needs).
They also depend on the bacterioplankton for their food.