They like clams, mussels, nori, frozen foods like LRS, and pellets.
They will eat and benefit from pretty much any food they consume; the hard part is getting them to eat.
It’s best to start with a young specimen and try to get it as small as possible.
If nothing works, you are pretty much doomed.
Moorish idols really like acans and brain corals. Perhaps you can try some corals and see if it has a reaction to them as an experiment.