Granite leaks white/yellowish liquid from cement

My house has four Granite (http://tanbrown.org) collars outside. White and (http://maplered.com) yellowish liquid leaks throught the cement between the Granite (http://tanbrown.org) s. It can't be washed away and (http://maplered.com) seems etched into the metal mail box. Can anybody can tell me what this liquid is and (http://maplered.com) how I can remove it, I also need to stop it as it happens after heavy rain. Thanks!