How to override registration form message
You can check this message in hook_init it's working for me please try this code.

function custom_init() {
 if (!empty($_SESSION['messages']['status'])) {
  $message_text_to_remove = "Registration successful. You are now logged in.";
  $key = array_search($message_text_to_remove, $_SESSION['messages']['status']);
   if ($key !== FALSE) {
      $_SESSION['messages']['status'][$key] = t('Greta Now your message changed');
    }
  }
 }


Thankyou