Indeed certain corals do and will eat vegetable matter. I am not suggesting throwing a head of lettuce into your reef but I after much reading about coral diets I cannot but help believe that they actually do ingest a small level of vegetable matter.
As you know reefs at times have much of what is refferred to as Reef Snow. In this mix are variuos levels of plankton,algae, detritus, fish poop and the like. Corals feed on reef snow so it therefore would make sense that particualte matter in the form of vegetable/algael matter make it into the digestive systems of both LPS and SPS corals.
I know that this may start some controversy but it will be interesting to hear what other reef keepers believe.