Is it a film or just changing colour? Foxfaces are any number of shades of black/brown/yellow/white/zebra striped throughout the day depending on mood. They're very skittish and mine will lose it's yellow coloring if I so much as walk too fast past the tank. I only see him yellow when I keep 3-4 feet away from the glass, or sit still for a while as I'm watching.
Foxface change color thought the night as well as create a thicker slimecoat when they sleep, as do most fish. Your's appears to be happy and healthy. Your foxface will have many weird behaviors. They are a very fascinating and curious fish to maintain.
It looks more like a film than a color change. It seems to have appeared since I put a new clown tang in the tank who is not very friendly. I'm concerned that its some disease that is showing up from stress
He is probably excreting his slime coat and some is still attached to him. You will occasionally see it floating in the tank and other fish will eat it as well. Nothing to be concerned about. This fish like mentioned above do some crazy things and are very skittish!!