I found this thread trying to solve a similar problem. searched on "delete greyed out user mac"
Accounts with Admin rights can't be deleted must convert to standard user to delete
Step one : log on using the Admin account you wish to delete
Step 2: navigate to users & groups under system preferences
Step 3: Click the lock to allow changes
Step 4 Unclick "allow user to administer this computer"
You will get a message that you need to log out to make this change
Step 5: log on using a different Admin account
Step 6: navigate to users & groups under system preferences
Step 7: click lock to allow changes
Step 8: highlight the standard user to delete
Step 9: click "-" to delete
Step 10: Decide on how much to delete of teh user acount, I selected all user files
I am running Yosemite 10.10.5
Accounts with Admin rights can't be deleted must convert to standard user to delete