Looks a bit like a thermostatic radiator valve without the head on
Yep that's exactly what it is.
Looks like a gate valve with some calcium carbonate scale.
and what looks to be some sort of dried landlord repair fluid.
and what looks to be some sort of dried landlord repair fluid.
Not that the landlord needs to be really broken before you’re able to see the benefits of using the repair fluid.
For slightly broken landlords, don’t waste perfectly good repair fluid on them.
and what looks to be some sort of dried landlord repair fluid.
Ahh yes. Landlord fluid.
Copper carbonate -- it is greenish. The white stuff in the front is paint, I think.
It'll pick up some copper oxidation on brass fittings.
Probably some sort of gate valve to shut off water to the unit. The crusty white ring at the pipe fitting is mineral buildup that can happen in plumbing.
The mineral build up can either be from natural condensation that occurs when it’s cool, or the fitting might not be, or might not have been, 100% water tight. As long as you’re not seeing significant water on the floor, you’re fine.
It also looks like there is a little old white paint or sealant on the valve.
