Square shower drain cover

I have a new marble shower in a new home. The marble around the drain is dark and discolored. It appears to me that water runs in around the edges of the square drain cover into the area outside of the round drain beneath it. I let it dry out and lighten to the original color, resealed the tile and grout, and after one shower it is back to the dark discoloration. The work was done by experienced contractors. I’d like to know if the problem is in the way the drain cover is installed.