Marble backsplash sealing
You are correct in that marble should be sealed before installation because it can absorb color from the grout. Luckily, your marble and the grout are very similar in color. You need to seal the grout, and now the tile, after the grout has cured. The time for the grout to cure should be on the bag of grout, usually 24 to 48 hours. Pick a sealer for marble and follow directions, there are many to choose from. 

I'm curious, didn't the contractor know to seal the marble first?