"Some were surprised to see a Muslim leader wearing a turban on stage behind Trump given Trump's remarks in the past about Muslims. On social media, Hassan was criticized by some for appearing at the Trump rally. Several years ago, Hassan left Hizballah - a group in Lebanon classified as a terrorist group by the U.S. government - and is now with a dissident Shia group that criticizes Hizballah, the Free Shia Movement. Last year on a TV show in Lebanon, Hassan called Hizballah "hooligans,"according to the the Middle East Media Research Institute (MEMRI), which monitors media in the Arab world." (thanks Basim)
PS This is a list of his legal troubles in Lebanon and his links to the Saudi regime and its apparatus in Lebanon.