Unique units might become unhappy with no way to fix it
They don't eat, they don't sleep, they only require gold. This one at the screenshot probably missed the payday or something, I can't think of any other reason for him to be unhappy. The spell which gives gold to a creature is not effective, his mood is still unhappy. So that's it, there is nothing players can do to make them content again.
Had a look into this and can confirm. It appears that these units do not have any need other than gold. This means if they ever miss a payday or for whatever reason get less wage than necessary they will drop immediately to unhappy.
Not sure why giving them the pay after didn't work though or why they missed the payday in the first place. Exporting to Internal Tracker :)