Algae on monuments of all stone types (granite, marble, etc) grow from the microscopic pores of the stone.It's virtually impossible to remove the algae from the pores.Cleaning the surface, even with chemicals, will not permanently remove the algae. It will return after only a few years.
Try this for relative easy algae removal from the surface of monuments:
1. Use a stiff bristle fibre brush to remove the larger parts. 2. Smear, with your hand, shaving foam (not gel) over the algae. Let set a few minutes until the algae softens. 3. Use a pad of florist styrofoam to rub away the shaving foam and algae. 4. Use clear water to rinse. NOTE - Experiment with a small area to check results before cleaning the entire monument. Work with small areas, approx. one square foot. Never, Never use a metal or stone tool to scrap or rub on a monument. Never use an acid based solution on a monument. (From a monument builder with 15 years experience.)