not sure, do you have sand in your tank, I do and sometimes they just have to spit some of it out during feeding.
Or maybe he just trying to get stuck worm out of his mouth. It could just have a piece of the worm stuck on his tooth, the clown knife has a large hook like tooth inside its mouth.
Or could be his gills burring from ammonia in the water, need more water changes maybe.
Or, yawing not sure. But I have never seen my clown Knifes just biting at nothing, that's a new one for me.
All I can think of keep a eye on him, Let us know what you come up with.