Turns out there's a "faceless" tag on here! In most cases, it looks like the facelessness is itself an indication of mind control, usually mindlessness and personality removal. If this character is already faceless, I think you'll have to make the posture do the work for you. Standing at attention, zombie walk, or leaning towards the source of hypnosis could all work, depending on what you want to indicate.
I did find this pic, where a slumped posture and some sparkly effects do a good job indicating what's going on.
hypnohub.net/index.php?page=post&s=view&id=216399