Most of NYC's sewers are combined sewers, meaning they collect both sewage from buildings and stormwater runoff from the streets in a single pipe and carry it all to a wastewater treatment plant. During times of significant rainfall or snowmelt, however, the increased volume of runoff entering the sewer can exceed the capacity of the system. In order to prevent a backup, the excess (including untreated sewage) is dumped directly into area waterways.
Some areas of the city, however, like almost all of southeastern Queens, have two separate sewer systems: one that carries sewage from buildings to a treatment plant and one that channels stormwater runoff directly to a local waterway. This prevents heavy volumes of runoff from overloading the treatment system and causing sewage overflows, but it also means that any pollutants in the runoff will be discharged into the city's waterways without treatment. Hence the warning on the storm drain above, which I assume was painted pink to call further attention to its message. (Other nearby storm drains were painted bright colors as well. Also, if you're wondering what the green and white dots are, here's your answer.)