Hello,
Looks like the cover for the fuel pulsation damper. The damper is on the passenger side fuel rail at the firewall end of the rail.
The cover protects the damper and a small screw that is attached to the diaphragm of the damper. I've read the screw often falls out but this has no effect on the operation of the damper, the screw is just a simple tattletale to show the system is pressurized. Guess the cover would also keep debris out if the screw fell out.
Hope this helps,
Jim S