Is this factory preservative, rust, or fouling?

[Androctonus]

Hi guys. This is on a new DD upper I just got. Just wanted to make sure this isn't rust. It came off easily with a q-tip soaked in solvent. I suppose it could easily have been factory preservative and/or fouling from factory test firing. Easy spot to miss after the test fire cleaning. I've seen Colt's factory preservative and it didn't quite look like this.

[26 Inf]

It came off with a q-tip soaked in solvent. Does it really matter in the overall scheme of things?

[Androctonus]

It probably doesn't matter, and it's just a flash hider after all. There was a little rusty stuff in the receiver extension too. Sorry, I know it sounds kind of silly. I'm relatively new to ARs and guns in general.