Thank you in advance for helping me to discover a remedy for this situation.  I have a friend who owns a hardscaping company who installed "tabby stone" as a pool deck.  Tabby is a cast concrete with oyster shells in it.  As you can see from the photo, many of the tiles have retained water marks around the edges and will not go away.  Grout has not been installed yet.  Is there a cleaner/solution that can be used to get the stains out using a nylon brush on a floor machine.  I don't have much experience with outdoor projects.