The Blizzard quest / achievement tracker will sometimes bug out removing an achievement , leaving behind a phantom entry. You can expand and contract it but the (1) won't go away.
Solution A :
/script RemoveTrackedAchievement(GetTrackedAchievements())
Is the easiest solution. This will get whatever's tracked and remove it from the tracking.
Solution B :
/run print(GetTrackedAchievements())
This will give you a number. Let's call it X
Then run
/run RemoveTrackedAchievement(X)
NB You could possibly use /dump GetTrackedAchievements()
Solution C :
WTF -> Account -> (Account Name) ->(Server Name) -> (Toon Name) -> config-cache.wtf
Edit out :
SET trackedQuests "v "
SET trackedAchievements "v "
And save.