I've looked in my book and unfortunately do not see any info on a Jefferson (or Jeff) or Mahala Mosby.
You may be aware of this, but in case you aren't, there is a Jefferson Mosby (born about 1825 and working as a Farm Laborer) who, along with the others in his household, are listed as Mulatto on the 1870 census and are found living in the Huguenot Township of Powhatan County, Virginia. Others found in his household at this time are: Mary I. Mosby age 28, Abner Mosby age 13, Adelaide Mosby age 10, Powhatan Mosby age 5, Junnie M. Mosby age 3, and Mary Saunders age 50.
Could this "Mary I." possibly be your Mahala? Do any of the other names ring a bell? I've looked in the book for the other names as well, but haven't had any luck. Let me know if these other names sound familiar or if you think this might be the family you're looking for and I'll gladly see what else I might be able to find on them.