He sounds like he is getting picked on, probably by the tang. They are pretty shy when first introduced to a new home. They are like mandarins, slow deliberate feeders that spook easily. Feeding them is only part of the difficult part. Some acclimate great and live long lives and some just don't. I have a friend in another forum that has raised butterfly's and moorish idols, pipefish, lots of stuff and his secret is feeding live foods. He even raises worms. But that's what fish can't resist, even finicky ones. He also uses a feeder for fish like butterfly fish.
I'd keep an eye on the tang.