I probably shot this carrier a total of 60 rounds. Cleaned it and put it in my parts bin in the same place that all my other spare carriers are and hadn't touched it until a few days ago. None of the other carriers have this issue and it looks very brand new. I'm wondering if this strange wear is corrosion, milling marks, or something else.

I never noticed this when I got it, but I never inspected it previously.  There is zero wear anywhere on the carrier and it is DLC coating (looks brand spanking new). It's in a weird area too. I have 3 of these carriers and this is one of the ones that looks funny to me. Now looking at some of the "inner" area of the carrier, I can see machining marks all over and discoloration that might be due to the ionbonding (purple, reddish-orange but not rust looking). The other carrier don't have this either.

I may be wrong but that's not actually a bearing surface on the carrier.

I'm gonna say it's leftover from the forging process and is an area that doesn't receive much if any machining.

If its ionbonded/DLC then it likely was like that when you got it.  Unless its got a rough edge I wouldn't worry about it