My mother is a convert. I recently found out that her biological father (who was not around when she was younger) is Greek Orthodox. Is it incorrect for me and my family to have any contact with him and his family? I know that when someone converts it is as if they are re-born and no longer part of those families. I am not sure what to do, because, this man still is my biolocal grandfather. is there any Halachic problems with contacting him?

As you said – though he is not related to you on a Halachic level he is still your biological grandfather and as such you can have a relationship with him, as long as he accepts you as you are.