Position river rocks around the pond in positions that receive plenty of light but not necessarily direct sunlight.
Spray or paint the rocks with a solution of seaweed fertilizer and water. Use the strength recommended on the packaging for most plants or a slightly stronger solution.
Mist the rocks with rainwater daily. The key to encouraging algae growth is moisture. If the rocks dry out quickly, move them to a shadier location.
Apply more of the fertilizer solution once or twice a week until the rocks have the desired appearance. After this, leave the algae to take care of themselves.
Set up a small tank with dechlorinated water and all the rocks you wish algae to grow upon.
Position the tank in a well-lit location or switch on a powerful aquarium light for 12 hours a day.
Add a little organic houseplant fertilizer to the water once a week.
Conduct occasional water changes to control the growth of free-floating phytoplankton, which compete with the attached algae for nutrients and light. Replace about 25 percent of the water with fresh dechlorinated water once a week before adding fertilizer.
Transfer rocks to the main tank or pond once they have sufficient algal growth. Replace each rock with a new one if you want a continuous supply of algae.