I was thinking...what if the person who wrote "HE IS MAR" on the wall wasn`t that men who knew Red John, but the hooker? In this case, the whole "he is mar" shouldn`t mean a name, or an occupation, as the hooker can`t guess those things. So, in this case, "he is mar" should reffer to a physical characteristic. He is marked, maybe? He is married (if RJ is wearing a wedding ring)?
I`m just trying to say that maybe Red John has something on his physical appeareance that makes him look different, or special.