In the Manustery at H-9 in Windurst Woods, talk to Kororo. She will say, "That spare (Insert Cardian #) there is but a newborny Cardian! So he memorizes anythingy and everythingy without reservation."
Talk to the Cardian that Kororo specifies. He will ask you to give him his new greeting. Choose one, it doesn't matter which.
When you come back some time later (after JP midnight), talk to Kororo again. She will tell you that the Spare Cardian you taught the greeting to would not forget it. Because you taught him the greeting, she believes you should take the responsibility for correcting it. She does not, however, know which Cardian this new Spare replaced.
The Spare Cardian you jokingly taught a silly saluation to has grown up .... and is unable to forget that greeting. Now that he's a full-grown Cardian, you better find him and undo what you taught him ... otherwise, he'll remain a walking joke.