Algae is a type of plant that appears in levels of Where's My Water?. However, if water touches it, it will grow bigger and in the direction the water is coming from. However, if Poison Water touches it, the algae will be cleared. Algae will be turned into Rock if touched by Toxic Ooze. Steam will pass harmlessly through it. Algae can also be a platform for balloons.
It also is part of the main conflict in Cranky's Story, the problem being that Cranky cannot eat his food because it is covered with algae.
Algae plays a main role as the antagonist in the episode, Rising Tide, where in its first form, is slug-like and gets Swampy to give it water. Later on, the algae falls into the river, allowing it to grow to an enormous size, taking the form of a giant alligator which captures Cranky. However, Swampy then saved the day and used a pipe containing poison water to extinguish the algae beast.