And cross that with this quote from Borneman
"It has been shown that many species of corals, especially Acropora, Stylophora, etc., may meet up to 150% of their energy requirements by their zooxanthellae alone. Yet, it has also been found that Acropora are vigorous consumers of zooplankton, nanoplankton, bacteria and other organisms which meet up to 70% of their daily energy needs."
Okay so they eat some food but its all microscopic
Another from Eric
"Mucus provides a nutrient broth of lipids, amino acids, sugars and other compounds that, in turn, provide abundant sources of carbon and nitrogen for microbes."
So they provide their own carbon and nitrogen . Remember these corals come from oligotrophic waters where there are no nutrients to pull from the water so they developed their own way to provide those elements in water which has nutrients cycled so tightly.
Another from Eric
"The bacteria present can work on particulate matter, dissolved organic matter, and even the mucus itself to potentially change some of the substances into forms more usable by the coral. Furthermore, the large number of bacteria may act as a “lure,†attracting zooplankton that can then by captured by the coral. One step further, the larger zooplankton populations then provide corals with waste material from themselves and from the animals that come to feed on them. Protozoan grazers of the bacteria (who also get a bad ‘rep', but actually provide another important food resource to corals) actually help to maintain high levels of bacteria in the mucus through their grazing activities. Finally, bacteria secrete a number of antibacterial agents which may provide a level of immunity to the corals. This is commonly known among us humans as “probiotic.†In fact, of 491 isolated strains of marine bacteria, both free living and animal associated, 126 produced antimicrobial compounds. The potential ability of corals to control the amount and composition of their mucus, and also to have it controlled by external factors, obviously has interesting implications in terms of coral immunity and health. "
Another from Eric
"The degree to which any coral feeds on bacterioplankton is species specific, and depends largely on whether they possess ciliary-mucus filtering mechanisms. All surveyed corals utilize bacteria as a SIGNIFICANT part of their diet, but some of the genera which depend to an even larger degree on this RESOURCE are Acropora, Pavona, Goniopora, Favites, Symphyllia, Leptastrea, Tubstraea, Seriatopora, Pocillopora, Montipora, Porites, Hydnophora and Turbinaria. Zoanthids are all very heavy consumers of bacteria. Soft corals and gorgonians also feed on bacterioplankton. However, compared to the stony corals, many do not produce similar amounts or compositions of mucus to enhance this ability . The xeniids and other soft corals with a heavy mucus coat, expectedly, consume more. Tubipora musica and certain small polyped gorgonians, including Mopsella, are also extremely proficient bacterial feeders."
So by what I gather SPS can provide all of their needs on their own but they are also opportunistic feeders.
