Does it look like it is extended its jaws out of its body - like it is yawning nearly? I have a lyretail anthias, and she does this with some regularity. I believe it is typical behavior, but I'm not sure. I can tell you that my fish does this, and she is a happy, healthy fish. It is probably typical behavior.
I got 2 dispars anthias that do that as far as I know its fine cause mine seem perfectly healthy as long as they're eatng and don't show any signs of stress