show text If special user is logged

I am not sure exactly what you are asking for, but, if you want to target a specific user ID, just do something like:

// set special user id
$special_user_id = 5;

$current_user_id = get_current_user_id();

if ( $special_user_id == $current_user_id ){
    echo 'special user is online';
}

You get the idea.