Looks like Euphorbia tirucalli, AKA Pencil Cactus.

And a not very happy one at that. Stems should be pretty rigid and upright, not sprawling over the side of the pot. Probably needs more light and some staking or pruning.

Agree. Like many plants in the Euphorbia family the plant contains a sticky sap, that if you get it on your hands can cause burning and inflammation and blistering and if you get some of the sap in your eyes, it can cause a reaction from a painful burning to temporary or even permanent blindness. (depending on the specific species of Euphorbia.)

IIRC the sap of the Pencil Cactus is well above average for the Euphorbia family in terms of causing painful burns. So if you decide to prune it as gardengal suggests, wear gloves, and safety glasses and be careful.

http://thomasosbornemd.blogspot.com/2010/09/i-recently-got-sap-of-this-plant-on-me.html

That droopy, cascading habit makes a lot more sense if a Rhipsalis :-)

Possibly a Rhipsalis pilocarpa, is it smooth or fuzzy or how does it feel?

They are also calling it Baby mistletoe cactus.

I too think Rhipsalis.

I grow this one. It is Rhipsalis pilocarpa, form minima.