What size is the tank you intend to put the idol in Bob? Idols need a lot of room to swim. That streamer on their dorsal is there to reduce drag so you can begin to see how much space they really need. Also, some fish will adapt in captivity and start eating corals. This is a mechanism that often gets triggered by malnutrition. Like Paul said, they eat the coral in order to get the nutrients they need from the symbiotic algae. The coral isn't the target it's food by proxy.