Window pane staining - How to remove from sandstone

[Stonetrade]

Hi -
I've been asked how to remove window pane
staining around the edges of indian sandstone flags.
Apparently the joints were grouted with a very wet
mix and the stones were not soaked before hand, and
the grout has seeped into the stone an inch or more
around each stone. the condition has remained
after several weeks of intermittant hot summer
weather and summer thunderstorms.

[Rich H]

Can you post a pic? It would be useful to see - could just be effluorescence?