grass, tree leaves, algae...
No, their mouth does not have the correct structure to do so.
Stonefly nymphs mainly feed on leaves and algae. They must be submerged in the water though in order for the nymphs to eat them.
Chilton's Eat Algae
Frogs do eat Algae but frogs prefer to eat insects but if all else fails they eat algae
pokemon eat algae probably
my pleco loves algae pellets.
Green turtles eat mangrove leaves. It is not just the leaves themselves which attract the green turtle, but epiphytic algae or invertebrates found on the leaves. Certain crabs have also been observed eating the seedlings and leaves of mangroves, such as the mangrove crab and the tree-dwelling mangrove tree crab.
Snails will eat any soft leaves close to the ground.
If you do not feed them (with vegetables, flakes, algae wafers, etc) at all, most will choose to eat algae before they choose to eat plants. When that is not available, they will turn to any rotting leaves or stems they can find. When there are no unhealthy or decaying plants, they will eat any living plant they feel like eating.
Some plankton do eat algae. Phytoplankton are a form of algae, and some zooplankton are consumers of phytoplankton.
Mosquito fish are opportunistic omnivores; so if the opportunity to eat algae came along, they would likely eat algae.