Should I be mad about this?

My boyfriend and I are long distance.. we are moving in together next month.. i was looking on his Facebook and see he hid his friends.. I was wondering why so I went to the one girls page who I do not like (she tries sleeping with everyone’s boyfriend) and low and behold.. he is on her friends list. We haven’t argued in a while so Id hate to start a fight over it but it also makes me uncomfortable and the fact that he is trying to hide it makes me more upset. How should I go about this.