you need A PH neutral cleaner. Non acidic. Unless theres a tough spot Id suggest water and clear dawn dish soap. Im not pushing brand names here but if you need a more agressive cleaner down the road stay with A Tile specific brand take a minute and read here.

And just for fun, go ahead and check to see that your marble has ENOUGH sealant on it: take a drop or two of water and place it in the centre of one of your tiles. Allow to sit for 1-2 minutes. If it STAYS BEADED, you are fine. It if ABSORBS into the tile (water disappears and the tile is left with a slightly darker surface colour) then you need more sealant.

Marble tiles often need as many as 3 coats of sealant. The tile installer will usually seal the tiles BEFORE they are installed and then twice more once installed.

Before using ANY cleaner on the marble, please ensure you have the required amount of sealant. And then keep the brand/bottle around because marble loves to being sealed as much as 3 times per year (depending on the brand of sealant...etc).