Why are database users grayed out and cannot be selected in the Database Users menu?
There are two possible reasons for a database user to be grayed out in Plesk at Domains > example.com > Databases > User Management:
A database along with its database user were created automatically by Plesk during an application installation (WordPress, Joomla, Drupal, etc). The message Used by an installed web app will appear next to this database user. These database and user can be removed only along with their application (Domains > example.com > Applications).
The database user is selected as a default user for accessing the database in phpMyAdmin: The option Use this user’s credentials by default for accessing phpMyAdmin is enabled at Domains > example.com > Databases > User Management > db_user.
With this option enabled, a database username will also be displayed in bold in the User Management menu.
To remove this database user, disable the Use this user’s credentials by default for accessing phpMyAdmin option.