I have located a great diy site for you concerning the care and cleaning of marble and also includes how to clean certain stains.
Muriatic acid for marble cleaning.
It is sold in one gallon containers and is used to clean masonry and as a rust remover which is what we will use it for.
The powr flite troubleshooter addresses some of the most common floor care problems cleaning professionals encounter and most importantly how to tackle them.
Muriatic acid is a corrosive acid that is manufactured by absorbing hydrogen chloride in water.
If the acid bubbles or fizzles it s marble.
If there is no reaction it is likely granite.
Muriatic acid is often used for cleaning brick and metal and for swimming pool maintenance.
Hydrochloric acid is available in most hardware store as muriatic acid.
The chemical is so strong it is known to etch stone and concrete and it can remove varnish and destroy many materials including plastics and fabrics.
Muriatic acid will not harm glazed porcelain though diluting the concentrated solution about one volume of the concentrated acid to two volumes of water is a good idea.
Rinse the area well with clean water.
One way to test is to place a few drops of the acid onto someplace inconspicuous perhaps a corner near the caulking.
Leave it for ten or fifteen minutes then rinse thoroughly.
Apply a few drops of the acid solution to the stone surface on an area about the size of a quarter.
One drop of diluted solution of hydrochloric acid is applied to the floor.
Do not inhale the fumes or get any on your skin or in your eyes.
Wait a few minutes then scrub well with a nylon brush.
Marble is a porous material and does not take kindly to just any old cleaner being applied to it.
Mix one part muriatic acid to eights parts water then spray or brush the solution onto the moldy area.
You might want to read through it and bookmark it for future use and care of your marble.