The pixies won't let you go free until you have completed some chores for them, and Sul Uin has one such task.
This quest is available if you have completed the quest Il Mheg, the Faerie Kingdom.
Starting the Quest
Talk to Sul Uin in Il Mheg to begin the quest.
Sow Everbloom Seeds
At each of the quest icons on the map, dismount and target the sparkling Destination, and then you can either examine or target it and use the Everbloom Seeds on it from the Key Items section of your inventory, or you can target it and use the icon to the right of the quest name in your list of active quests.
Speak with Sul Uin
Go to the quest icon on the map and talk to Sul Uin to complete the quest.