First off, are these marks fresh? How can you tell they’re not from another source? Has this person shared their medical history with you?
From the point of view of a formerly employed person with, well, let’s just say “mental wellness issues”…
Unless this person has reached out to you or it’s affecting their work, it’s none of your business. Really. I can’t stress this enough.
Mental illness is a career-killer. Even word of mouth can kill opportunities for advancement in a small industry. Even the nicest HR dept. is not the friend of the employee. HR are like the cops. They are there to protect the company, any help they offer is only in service of that goal alone.
You don’t know what demons your employee is wrestling and you are absolutely not equipped to deal with them. Perhaps they are dealing with things by channeling their energy into their work, giving them a felling of self-worth by proving to themselves that they are as good as anyone else. Perhaps they see their job as a sanctuary from their other issues. Or perhaps they’re on the way back up, putting it all behind them, rebuilding their life.
I know you’re wanting to help because you really want the best for your staff. In this case, the best you can do is be even-handed, supportive of all your staff and create the sort of environment anyone who needs help feels able to reach out, if they need it.
Besides, they might just really be into that style of edge-play and you’ve misread things.
You sound like a good boss. Sit back and let them be a good worker.