Actually, I'd be upset if I thought someone was self-harming. I'd be in bits thinking that they probably weren't getting the support they needed/could do with. I'm not sure I could absolutely ignore it - I'd feel I was colluding with a kind of silencing of any (mental) pain they were in.
But I wouldn't know what to say or whether to say anything. I know that when I;ve felt down, even desperate, what I wanted/needed was support ... but, at the same time, if someone had offered me that support in some unexpected way, I'd have lost whatever control I had.
I think I'd have gone the route of asking, in a very private way, about the recent mark, in a way that left it open to be described as a cat injury, but (hopefully) in a way that suggested I;d listen/be aware. And leave it to her to show me what she wanted me to do.
In a way, you've done that, and she's told you how to take it.
REalistically, you are probably not in a position to offer anything else. So, perhaps you just have to leave it there.
I'm assuming that your worries are for her not about her competence. The two issues - mental well-being and job competence - don;t sound connected at all in this case. That leads me to conclude you are worried about her.
It is hard to not leap in, though. A friend of mine self-harmed for a long period. She was talking about things with a counsellor, and I guess my position was just to be a friend and hope things would get better.