The two main concerns about using carbon in a saltwater system is that carbon often leaches phosphate into the aquarium, and that it removes necessary trace elements needed by reef animals, particularly corals.
The Phosphate Leaching Issue
As far as phosphate goes, you want to remove this element from your tank, not add it.
A high phosphate accumulation in saltwater aquariums can lead to aggressive hair algae blooms that are difficult to get rid of, and since some brands of carbon may leach phosphate into an aquarium, you should test the carbon you are using, or going to use, for leaching. If you find any relevent traces, change to a different brand.