I rarely see this in my cories, but in my opinion it was originally a wound and is in the process of healing. If you are new to cories, you might be surprised at how active they can be. I noticed you have some lava rocks in there, it's possible that in all their daily hustle and bustle it may have crashed into a sharp edge on that lava rock. I used to keep a couple of pieces like that myself, but no longer do.
I don't have any direct evidence of lava rock-induced damage, but I haven't seen any other wounds like that after taking it out.
If it doesn't look to be getting any worse, I'd say keep doing what you're doing. Keep the water clean and keep an eye on it.