They may be ‘smiling’ all the time, but their real emotion needs to be determined from their fin and tongue language. Pupil stuff depends on context; normally it’s a way to show you’re annoyed/give a warning/show shock or anger, but it can also be done when very excited to make it clearer you’re looking at the other person. Vocalizations are also more a part of their expressions than they are for humans, with some facial movements not hitting the same or meaning something else if you don’t make the right noise.
Fun fact: that big facial gland was a lot more useful back when they were living in salt water and had to expel a lot more excess salt. Now, it mostly just helps their endocrine/ excretory system with getting rid of concentrations of spent copper and urea (from their blood) and other contaminants that seep in through their skin. And for crying, of course. Much like humans, emotional mucus discharge is similar to emotional tears in that it is chemically different, containing stress hormones and being thicker. The viscosity helps it bubble when pushed from the gland, which makes their intense emotion more visible to other Kixeli.