Current limiting from a USB supply
Would a simple PTC/Re-settable fuse work?


PTC's are very slow. This is often used in the host to prevent board damage because they are cheap. If you want any selectivity (eg: your protection tripping first) you'd want something faster.


  Or something like this current limiter from Wikipedia.


A current limiter will prevent you from tripping the host protection, but will also cause brownout on the microcontroller or ws2812.


  Or should the supply be cut completely? 


Yes, you want the power to trip immediately without your microcontroller browning out or the host protection to trip. Prevent the USB connection to be lost.
Luckily you can get current limited power switches. Take a look at mic20xx for example.