Moorish idols aren't the black box they once were. Had two in the past. One for 3 yrs and one for 5 yrs. Both rehomed to fish only tanks. They are just NOT reef safe long term. Will start picking at and eating more and more types of corals as they grow. Only found a handful of corals that safe from mature adults. The secret to moorish idol success (at least for me) was frequent feedings and LOTS of nori. These fish have high metabolic rates and in the wild graze almost constantly on algae and benthic inverts such as tunicates, sponges and anything else they can get their mouths on. They are not picky eaters once settled in a tank, but need food many times throughout the day.
Here's one of mine that was eventually removed from the 500 gal sps tank.