Marble will dissolve a litle if the ph is less than 7 and the marble will also leak phosphates just like the reef rock.They are basically the same chemical composition just the marble is more compact and cristalised.I would try thoose porrous volcanic rocks because they look similar to the calcium carbonate rock and they can also host bacteria for filtration.And its a natural look because manny reefs are growing on atols made by volcanic rock but be carefull as the chemical composition of that rock varies from one volcano to another.Somme might contain harmfull substances that could leak in the water.Otther options would be man made porous ceramic (burned)thats totally inert and can be made to look just like the live rock but without the risk of leaking phosphates or cement made rocks with cuartz sand in it ,preferably a ceramic, white cement.