In principalities such as Monaco and Lichtenstein, the ruling princes are known as "His Serene Highness," in comparison to the "His Royal Highness" title in the UK. The heir apparent to these two contemporary principalities is known, simply, as the "hereditary prince."

Source: http://www.liechtensteinusa.org/page/government https://www.vogue.com/article/prince-not-king-of-monacohttps://www.vogue.com/article/prince-not-king-of-monaco