A pH neutral stone cleaner is what is needed. Switch to pH neutral body washes an other personal care products as well. No bar soap. And squeegee or towel dry after use.

Thx for your prompt response. Can you provide me with the name of a product and a store that might sell it?

I had put off cleaning the little river of soap scum on my marble for - - - - years. This because I couldn't find the right magical substance in a bottle or can that would do it without injuring the marble. THEN ONE DAY I happened to have a plastic item in my hand that I was throwing away. So I broke it in pieces and took one of those pieces with a fairly straight hard-ish edge and thought maybe this will do that damn soap scum in the shower. IT DID NOT EVEN TAKE 2 MINUTES TO GET IT ALL. No water except to rinse no chemistry no fancy spendy products nothing but that little hunk of plastic and a little bit of elbow grease using it like I might a razor getting paint off glass. And it got it all. no damage to the marble.