I don't know what would cause that, but if I've understood Rohloff's exploded diagram correctly that's the edge of a seal. So, other than doing as the name suggests, not a critical component.
If it were mine, I wouldn't be unduly worried if it was running smoothly, though I would want to investigate further. I'd probably take the axle ring off as if I was going to change the cable (And do that at the same time if it was anything like due - Rohloff have a YouTube demo) That will both give you a better look and a chance to clean it up thoroughly. If it all looked clean then I'd re-assemble with a generous smear of something to inhibit corrosion (I use marine grease for these sorts of jobs, but there's plenty of options) and monitor it for leaking. If there was any corrosion in that recess, I'd be a little more concerned. The main thing I'd want to satisfy myself about is that the corrosion was from the outside in, rather than the other way round.
SJS used to list the parts to replace the seals, originally along with the tools to do it, then as a workshop job and now it's not listed at all. I don't know if this is because of the way the hubs have changed, or something else. My older hub sometimes leaks, or more likely lets water in and then lets oily water out again, but only after a particularly wet ride, I really ought to get it sorted, but in the meantime I give it a flush and fresh oil when it happens.
If you're concerned about internal corrosion, you can have a look, I'm not recommending it, but the instructions are here
https://www.rohloff.de/en/service/handbook/speedhub/workshop/gear-unit-replacement
