The person already has a posted shift outside the schedule you're looking at. You can see it, but you can't edit it here.
See where the shift is
Hover over the faded shift.
The Scheduled Elsewhere box shows the community, time, and position.
Edit the shift
Different position: open the position menu at the top of Schedule and tick that position.
Different department: pick that department in the department menu.
Different community: switch community in the menu at the top of the left sidebar.
Click the shift and make your change.
Good to know: If you don't have access to that community or department, ask its scheduler to make the change.
Common questions
Why show it at all?
So you don't double-book someone who is already working.
Do draft or open shifts show here?
No. Only posted shifts assigned to that person.