Eoin,
If you are speaking of the small branch on the left. I would slice it off with a razor blade (cleaned with rubbing alcohol) as close to the base. It looks like it could be rot, but either way you will stop it from moving further down and into the caudex. This small branch removal will not effect the look of this plant.
With using the potting soil in with your mix for soil. Make sure that you allow it to dry back well before re watering it.
At the same time it would be worth sending a photo and note to the seller of the plant and effected branch. This is only a safeguard for yourself. Not saying anything is wrong with your plant, but worth having it documented.
Rick
