That looks like either adhesive from where it came off the case or perhaps the screw was lodged there for a while causing the issue. Does it seemed burnt? If so, see if Hisound will send you a new battery and refer to this thread. If not, just put it back and call it a day. Unfortunately, most manufacturers frown on home repairs.
Ignore all comments on parts cost. Yes, they actually make money on it. If it's sound meets your approval for what you paid, be happy.
It didn't seem burnt, just very grainy. It was probably adhesive and until a problem rears its head, I think I'll leave it at that.
Not too concerned about how much parts cost them. I like the sound.