Replace chrome toilet shut-off valve
The chrome pipe and valve are a unit. The chrome pipe is sized to fit over 1/2" copper and then soldered at its end. The copper likely extends a fair bit within the chrome pipe, and it's not easy to find compression fittings for the chrome pipe, so your best bets would be to repair this valve or remove and replace it.

To repair it, you would shut off your house water, remove the valve handle, remove the packing nut, replace the packing material or any other worn parts, and then reassemble.

To replace it, you'd shut off your house water, remove your toilet line, drain the water from this area (because you can't melt solder well when there's water inside), and heat the joint with a torch. Once hot, use pliers to wiggle the whole pipe/valve assembly loose. Make sure you grip at the valve end - if you grip the chrome pipe portion, you might accidentally crush it around the copper pipe within, making the whole thing much harder to remove (guess how I know!). You can then replace it with another similar kit, or use a new solder or compression valve. I'd recommend a ball valve, aka quarter-turn.