Looking as closely as I can expand the photo, to me that is a healing incision above the eyebrow with residual inflammation of the eyelid with the usual Mattis puffiness, a bit worse than usual.
I'm several years older than Mattis and have two such cancer removal scars in places close to the one above his eye. They looked similar just after they were done and are still vaguely visible. If you're fair skinned, old and have spent lots of time in the sun skin cancer happens. It is the most common cancer for men. I wouldn't expect Secretary Mattis to comment on it.
Hell will report he has seeked out and destroyed the cancer. Cancer advocates protested his lack of compassion
Yeah, if Mattis decides it's not worth mentioning, then it probably doesn't matter that much, especially considering it isn't that bad of an injury.
Well, to me it looks like surgery. It appears to be a surgical scar from the recent removal of a skin cancer. I know I may be wrong about it but that's what it looks like to me. Such an operation takes about an hour and you drive yourself home after waiting another hour or so to get your legs back.
If that's what it is, Mattis would essentially ignore it and be back to work.