Damp surfaces on decks and pathways can remain slippery and dangerous even after they are dry because algae begins to form wherever water settles. Shaded pathways are particularly difficult to keep dry and they are often times the most heavily used.
Your first step should be keeping your path or deck as dry as possible. Your second should be renting or purchasing a power washer. A power washer shoots a stream of water at a high velocity which will clean many porous surfaces of dirt, grime, oil, and in this case algae, without damaging your stone or wood. After using the power washer it is also advisable to run diluted bleach over your stones to kill any remaining algae spores. You may need to repeat this process every few months but consider it an ounce of prevention.
And don’t forget to use your safety glasses.
Power Washing Decks [DIY Doctor]
>Pressure Washing Driveways [Ultimate Washer]