I hate to ask for an ID without supplying a photo, but my camera has done a walking trick.
Shouldn't be too much of an issue though, because the plant I have is quite unique.
I was getting ready for a meeting last week, and walked into the meeting room a few minutes ahead of the others. Someone had bought a zygo to 'decorate', and growing alongside the zygo in the pot (either from seed, or a stray rooted fragment) was a cactus that was basically just stem a little over one millimetre in diameter. It has the softest of hair growing (individually it seems) from what are essentially invisible aureolae, and it branched every 5cm or so. The overall form was very Rhipsalis-like, but I didn't think that any grew so fine. Perhaps pilocarpa? My impression was that pilocarpa wasn't quite as narrow as this plant, and that it was more branched, but I suppose that these characters might just reflect growing conditions.
I snaffled a small piece, but until I find my camera I can't take a pic.
Any ideas though?