Crushed marble may work fine in a freshwater setup, but personally I would not use it it saltwater.
It may look good, but will not aid in buffering the tank as other substrates will.
Aragonite, Puka Shells, Reef Sand, Live Reef Sand, even Crushed Coral would be a better choice in my opinion.
When it comes to decorations and substates, someone once told me " never add anything to your tank that didn't come from the ocean".
HTH
Brian