Data Validation Conditional Formating

I have a shared spreadsheet where coordinators enter staff codes for particular tasks they would like them to do (it's not a timesheet, just a future planning tool). I have set up a data validation to warn them if they enter a staff code that is not in the list, however if someone leaves after their code has been entered, the validation doesn't re-check so the now-invalid code remains happily in the spreadsheet. I need the cells to highlight if the staff code is not in the list.

Does anyone know if this is possible?

Currently, I need to search for each code as someone leaves and manually remove them from the spreadsheet and then ask coordinators to re-enter new values. It would be great if it would just flag them in red when I remove them from the staffing list and coordinators can see if there is (now) an error that needs fixing.

Not sure how to attach a file as it's not 'link'able with a URL, so here's screen shots:

user interface

formulas to calculate the total hours based on the task type

conditional formatting to highlight the table on the right, or when someone does a seazrch for one particular staff member using the box below the 1st table