On Super User, ui is a synonym for user-interface. On Meta Super User, they are two separate tags. That doesn’t make any sense. I recommend making ui a synonym for user-interface on Meta Super User.
I just discovered that this question is a duplicate of Can the “ui” tag be merged with “user-interface”?, but that one says that it’s status-completed -- but it isn’t. (Perhaps it got reverted in the intervening two years.)