Cleaning sandstone?

[Fluffycloudland77]

Our windows have sandstone windowsills, or beige concrete but it's got green algae staining on it.

I can't find anything to clean it though. What can you put on it to kill the algae?.

[irie]

We have a sandstone patio that's always going green, DH jet washed it... not sure that's an option for your windowsills though