I got Serafen to speak and then his speech bubble went away. Did "A Sorcerer and a Gentleman" > Erroneous bubble appears > Get Crookspur slave quest > Go to Crookspur and buy a slave, talk to slaver guy, release slave > Go talk to Huana up north about slaving, agree to kill slavers for them > Immediately after conversation ends about slaves/Leaden Key, Serafen accuses you of slaving and the bubble is gone. Went back and killed all the slavers without talking to them first and Serafen is still bugged. It seems that doing something negative is the only way to kick that bubble.