Q.Growth On Christmas Cactus
Found on a young plant

Certified GKH Gardening Expert
WOW That is cool!!! If another cactus was nearby, then this is likely the case: The other cactus has grafted itself to your plant!
Usually, this is done, by hand, and purposely. I don't think that this is the case, here. You can remove it and grow it as its own cactus, or you can leave it as a strange novelty.
Quite interesting!
https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/grafting-cactus-plants.htm