“Off him” indicates that the camera is on a character who is reacting to something nonverbally. It can also be noted as “Off her expression…” or “Off his [emotion].”
This term is often used to motivate a scene transition.
I love you, but I have to tell you the truth.
I love you, too! Nothing can change that.
Joe removes a convincing mask, revealing his alien face beneath. Off her confusion...
It can also motivate another character’s response to the noted character’s reaction.
Lucy, please say something.
(off her silence)
I thought you’d understand.
Lucy removes her own mask, revealing her own alien face.