What do you think?

So there’s this chick that my husband went on a date with when we were broken up that started messaging my husband around the time our son was born, I had no idea that this was going on and he won’t admit that they’ve been Facebook messaging for some reason... I’ve had to find out for myself when I went through his phone last night. I don’t remember much (because I was skimming and trying to not to cry, didn’t work by the way) except them asking about each other’s children and her saying she missed him ( because we moved states). He only admits talking to her on Snapchat once because I saw she sent him something, but I didn’t see it ( it’s always her initiating the conversation). What do I do about this? Do I send her a Facebook message telling her to back off? (Oh and by the way my husband finds nothing wrong with talking to her or anyone he wants especially single women.) If I do I don’t want to seem crazy so how do I word it? Please give me your input. please help.