bp-custom.php:
<?php
function cltgeeks_bp_core_get_userlink( $link, $user_id ) {
    $displayed_user = bp_core_get_core_userdata( $user_id );
    $oldtitle = 'title="' . bp_core_get_user_displayname( $user_id );
    $newtitle = 'title="@' . bp_core_get_username( $user_id );
    return str_replace( $oldtitle, $newtitle, $link );
}
add_filter( 'bp_core_get_userlink', 'cltgeeks_bp_core_get_userlink', 1, 2 );
?>

members-loop.php:
<a href="<?php bp_member_permalink() ?>" title="@<?php bp_member_user_nicename() ?>"><?php bp_member_name() ?></a>
or
<a href="<?php bp_member_permalink() ?>" title="@<?php bp_member_user_login() ?>"><?php bp_member_name() ?></a>