The short answer is yes!
During the warmer months, especially if your pond/waterfall is in full sun it is normal for algae to grow. Some people like this effect, saying “it makes it look even more realistic!” however if you don’t want that look you will possibly have to upgrade your filtration system, to minimize algae growth.
Having water plants in your pond will help reduce algae growth as they use up the nutrients before the algae can. There are some pond chemicals in the market that are made specifically for this, that are safe for fish, plants and rocks.