The title is sayyid (سيد) or sharif (شريف). I think the former is more commonly used in modern times. They're rather thick on the ground in the Hejaz and in Yemen. I'm sure over the years there have been spurious claimants, but I can't imagine how easily someone would get away with it, as Peninsular Arabs are as touchy about lineage and authenticity as a Habsburg. On the other hand, if a ruler claims such descent he would have the means to silence opposition, I would imagine.I’ve never heard that but many people certainly claim descent from the Prophet, and it earns them a special title which I am forgetting right now.