What surface is grouted (Tile or Stone, Mable or Granite)?
If the surface is ceramic tile or porcelain, or granite, try TESTing with a mild solution of 50/50 vinegar and water in an inconspicuous area. If that doesn't make a change, move up to full strength vinegar. I suggest dampening (no standing water) the joints prior to application, and agitate with a soft bristle brush.
Get back to us with results. There are stronger products, but we want to start with the least aggressive.