Saleucami Fleet Action Mastery achievement is bugged and stuck in my tracker list as well.
When I "completed" the achievement, I did not get a pop-up, though the achievement is listed as complete and all points are correctly tallied.
And, a further note - the date stamp on the Saleucami Fleet Action Mastery achievement now says whatever the current time is, i.e. every time I click on it in the Legacy window, it updates the timestamp to the current time.
It seems as if the pop-up not firing and the lack of removal and the time stamp are all related, like the achievement is not firing all the processes that should happen when completed.
Exact same thing for me.