php return username of currently viewed author profile

On an author archive page (assuming the plugin uses the core archives) get_queried_object() will return the WP_User object for the author. Something like:

$author = get_queried_object();
if (is_a($author, 'WP_User')) {
  var_dump($author->data);
}

You should see the login, nicename, and display name in there. Use what you need.