Ahh I have that same Mac. It's in sorry shape, but I rescued it from a closing computer shop for $5.

It is more than likely a piece of that greyish inner bezel (behind the clear pinstriped bezel on the front). My Indigo is literally falling to bits because that brittle plastic is just disintegrating. One of the CRT mounting screws broke off and is loose near the CRT, so it hasn't been powered up in a while.