What do I do about his ex?

I've been dating this guy for about a month now and I really care about him and I know this sounds silly but his ex left a message on his wall on Facebook telling him to let her know when he's off work so they could get plans underway. It sounds like she's planning on hanging out with him and it's been really bugging me. I know it's his ex because she has him tagged in her pictures, should I ask him about it or just let it go? I can tell she still has feelings for him and I don't want to stress him out so if I should ask him about it how do I go about it without sounding bitchy?