D2, water and a soft brush
What is D2
A biological cleaner. When in doubt, just Google! Here you go . . .
My friend use Spray and Forget on her parent's stone and it worked great.
Bring a bottle of Simple Clean and a brush to help clean it up.
This may help: https://dengarden.com/landscaping/How-to-Clean-a-Headstone-the-Proper-Way