Here are some instructions from Apple how to change your name showing!
I tested it on High Sierra, but do not have Mojave to test
Change the full name of a user account
A user's full name can be changed by that user or an administrator
account. The full name doesn't need to be the same as the account name
or home folder name.
macOS Sierra, El Capitan, Yosemite
- Choose Apple menu () > System Preferences, then click Users & Groups.
- Click
, then enter an administrator name and password. - From the list of users, Control-click or right-click the user that
- you want to rename. Choose Advanced Options from the shortcut menu.
- Change the name in the “Full name” field, then click OK.