I’m not familiar with the term mirror empath. It’s interesting to learn about, thank you. Does it mean someone highly empathic who not only feels the emotions of others, but also takes on their moods?

Most all people have empathy. When people smile at someone, they generally smile back. The same goes for negative or neutral responses.

The clerk who is cheery with the prior customer and then stone faced and silent with you is a strange occurrence. Where I buy groceries, the clerk says to nearly everybody “Have you found everything you need today?”. There is always some greeting to each customer. I’ve never had one be silent and sullen, and I’m generally putting out a neutral mood while placing groceries on the conveyor belt, using a lot of energy unloading the cart.

You say you are a regular. Was there ever some bad interaction between this clerk and you that would cause them to not want to greet you?