It is Austrocylindopuntia subulata (Eve's Needle). It does that to its 'leaves' when it is distressed - not sure where you are, but if it just got a lot warmer in the greenhouse it might a response to that, or if it started getting a whole lot of extra sun, that would also cause it. I'd show you a picture of mine who just did that before it put on a whole lot of new growth, but unfortunately it is a plant I look at a lot, but for some reason do not take many pictures of.
The grayish base is likely the woodified base of an older plant that was either cut off or died off - these things rot easily if wet in cool times.
The Eve's Needle really does not like wet feet, especially when it is cool. So it is a plant to keep pretty much dry in winter even if it looks a little unhappy.
The little plant in front of it looks like a young form of cylindropuntia fulgida (jumping or chain fruit cholla).
When I saw the picture I said to myself hmmm that pot could have come from me. My jumping cholla jumps a lot and I have many volunteers in bunches of pots crowding in with the plants that are supposed to be in the pot... in the monsoon storms we get here in summer in Arizona you can see small chunks of this plant get blown around. Watch those spines on the cholla they have little barbs and if you get stuck you usually get a section of cactus with it.... not fun to remove.
Cross posting with Tarev, but as you can see I agree with his ID!
