For the Hanar Diplomat mission, I do not remember completing it. But it is marked grey (typically when complete).
I have now finished the Citadel story mission where you
save the Citadel from attack
Why is this mission grey like I already completed it? Can I
still get Kasumi?
You can't get Kasumi as a squad member, only a war asset, as far as I know. Grey missions are considered completed, orange ones are not yet completed. It is unlikely you will be able to restart a mission that has been marked for completed.
Quests may be marked as completed (grey) if they are no longer possible to complete.
