I was on a motorcycle trip once in Central Texas back roads, A thunderstorm came up and drinched me. I came on an old Cemetary, that was kept up but didn't look like anyone had been there in a while. I got some dry clothes out and changed behind a large tombstone. I usually would get the "hebi jebis" in a place like that but after I changed I walked around a while and read. This may sound strange but I got the feeling that I was welcome and that it was ok, like someone enjoyed the company, kinda hard to explain, but I felt welcome and at peace there!
We used to clean my G G Grandpa's tomb in Louisiana with a mild bleach and soap solution, Dont think it was marble though, as it was whitewashed, probably concrete with stucko over that then painted.