As I posted earlier today, this plant is indeed an X Graptoveria which was developed by Robert Grim (note the correct spelling) of California. It appears that there are a lot of "unnamed hybrids" out there which were developed by him. And the one's which were named to honor him, X Sedeveria 'Robert Grimm' and this Graptoveria, both misspelled his name. We continue to use the misspelling, since this is the name which has been in circulation, until we are told it has been properly published with the corrected name.
By the way, another hybrid developed by him is X Cremnosedum 'Crocodile':