He could quickly revert if he wasn't in the middle of changing in the first place. Well, it wasn't so much a change as a momentary unwillingness to lie to reality. In either case, he was busy squinting below, wondering if he was actually seeing movement. A fog horn blasted from the ship, so he knew it was down there now. The distraction was enough to not see Kaylee's initial disturbance of the fog until it was too late.
When movement did catch his eye, he panicked for a brief moment, feeling that sharp rise of adrenaline, the warmth of his blood traveling through his body. The glamor went back up, but there was no metamorphosis. Suddenly, he just turned his head to look at the woman that stopped, and his ears were normal, his eyes were less vibrant, his skin didn't have that same glow. Just a normal, if above-average attractive male.
Still, he knew that she had to catch a glimpse by the way she was staring. The defiant part of him thought, Good, I shouldn't have to hide who I am, anyway. The rational part of him knew that there could be consequences, but in the end, he just locked his eyes on her, feigned a look of confusion, as if Kaylee were the weird one for staring.