What is this part called?
That's part of the derailer. Most SRAM derailers have a piece like that. They're not really supposed to come off and float freely but sometimes they do anyway. There's not a specific name for it, but if you wanted to buy a new one for this X1 rear derailer, it's part of SRAM pn# 11.7518.014.000, "XX1/X01/X1 Rear Derailleur B-Bolt and Limit Screw Kit".