That is just new 'arm' growing, and it will grow more of them White appendages are air roots, and are quite common on the plant that was originally used as a rootstock. It is most likely pitaya = dragon fruit = Hylocereus undatus. It will grow easily, just be aware it could get big. And very prickly !:)

Rob - it is actually very common for rootstocks to continue growing, and well. They are much more vigorous plants than ones grafted onto them. That plant could potentially grow to 20feet - obviously under good conditions. That means lots of light, warm temps and best if it has something to climb on. It could be tied to some support, otherwise gets quite all over the place. It could die because of overwatering - same as most succulents. It is more common for grafted plant to die than rootstock.

I stand corrected. I have never bought a grafted plant.

Rob, np. I never had a grafted plant either but I did had Hylocereus undulatus I got rid of because it gets too 'wild'-big, and it is so prickly :)

When the grafted scion was knocked off the plant lost its growing point - so it will grow another - happens all the time. Same thing is happening with this Myrtillocactus geometrizans - another failed graft stock. Your shoot does look a little etiolated, though - needs more light.

And aerial roots are especially common on climbing jungle cacti - this Selenicereus growing up my garden wall has got stacks of them - uses them to grip and support the plant.

Thank you all for the help! I'll be sure to share updates if anything exciting happens.