This is termed the "B-axle Cap C-ring." It's a split ring and can be removed by widening the split and prying out. It retains the rest of the cap assembly. Shimano doesn't specify this as a separate part number in their technical documents, but rather lumps it into the "B-axle Cap Unit". You may have to purchase the whole assembly to acquire this ring.
On page 18 of this Dealers Manual, one can check out the directions for removal of this assembly. In your case it may be possible to snap the ring back in if there no damage that has misshapen it or it's seat in the cap assembly.
The inner bolt will have to come out to relocate the ring.
Un mount the derailleur, look at the back side of the bolt where the thread that screws into the frame is, there may a circlip there that retails the bolt (this was the case on an old Shimano derailleur I had lying around).
If you remove the circlip the bolt will come out which will enable you to relocate the ring. Be careful though because the B-pivot spring is in there.