BuddyPress.org

>I still can’t seem to figure out where that @username went.

Look in this file, hopefully in [your-theme]\buddypress\members\single\member-header.php

Look near the top for bp_displayed_user_mentionname();

btw – you should not use ‘admin’ as a username.
https://codex.wordpress.org/Hardening_WordPress#Security_through_obscurity

http://www.wpbeginner.com/wp-tutorials/how-to-change-your-wordpress-username/