First, I need to correct something in your reply about my lineage. Rachel Clayton Falls was the grandmother of John Clayton Faries; not his mother. Her daughter Mary (Polly) Falls married Robert Miller Faries, and were John Clayton's parents. Just want to make sure we don't confuse the matter any more than it already is.
It appears that your connection to the Claytons was with either Robert Arthur or Benjamin. Do you have any information on their spouses? Even if you only know their given names I can cross-check it with the information I have on the Claytons. I know that Rachel Clayton had at least two brothers: George III and Lambert. I have some information on their descendents. We might find a clue there.
I am sorry to report that I am the person who put the story out on the internet that Moses' wife Mary was a Catawba Indian. I received this information from my cousins in the Arkansas branch of the family when I first started genealogy research about 13 years. At the time I had no reason to doubt the story. Researchers from the SC branch of the family were all very surprised by this story. Do date, no one has found a shred of evidence to support the claim that she was Indian. The story may have gotten twisted up with the fact that John Clayton Faries (by that time he had changed his name to Farris) married Lavicia Cranfill, who we know was half Cherokee. There may be some truth to that part of the story about Mary returning to SC after the death of Moses. Hard to say. Through my own research I have debunked two other tales about John Clayton Faries, one of which was slanderous in nature. Because of that I have taken all info received from the Arkansas branch with a large grain of salt. It seems that John Clayton may have had an enemy in the family tree who liked to spin tales about him.
Have you been in contact with Dan Troublefield? He is a very thorough researcher and is the person responsible for untangling decades worth of bad research by genealogists of previous generations, and for getting the York Co. Farris line in order. He probably can answer your question about the Alexander who married Mary Forbes.