Wow! I should have all mine together to on a pole! Very nice everyone! I am still looking for a pure white one. Paul, how in the world do you do that with images? I like it. Mike

Im late but this just happened to mine and this is the only post I could find that looked like it, except mine only had the white mold. Yesterday I did my routine poke check to make sure everything's good and It was perfectly fine, today I walk past it and noticed it had mould all in the cracks of it, I pushed a spine and it cracked open and was full of mush. Cant figure out what caused it either, had it for years and it survived a mealy bug attack and everything. Now I'm on a mission to find another one...

Check first if it is scale; moisten cotton ball or q-tip with some rubbing alcohol. If scale, it should wipe off. If it doesn't, it is likely something else. Could you get a close-up of the white 'stuff'?

Austrocylindropuntia subulata is the accepted scientific name. Not sure what is meant by temporary leaves, IME they last for at least a year. Easy to grow; needs good drainage (is there drainage hole in your container?); substrate needs to dry up between watering. Less or no water in winter, try to keep at temps no lower than approx. 40*F (it will tolerate less, but has to be very dry). Needs good light as most succulents do, and will grow few trunks and branches. Easy to propagate from offsets. Your plant looks quite etiolated, it really needs more light. It should be 'chubbier', and stems/trunks should be able to support themselves and grow more upright. I wonder about those white specks on trunk?