A few years ago we had "guest" users ( non registered ). Then we went to having all users register.
It's been a while since I've seen a green username, but in this case I think it's because this username is now deleted but the posts remain. The system now thinks it's a guest user.  We do not delete the posts when we delete a username. We rarely delete a username. Most times we get a request to change usernames, not delete them.

At this time I should point out that one should give careful consideration when choosing a username as well as think about what they post on the internet. It's for the world to see.