frog spit

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A foamlike aggregation of small aquatic plants, such as green algae, on the surface of a pond.

from The Century Dictionary.

• noun A popular name for various filamentous freshwater algæ, especially species of Spirogyra, which form floating masses.
• noun The frothy substance secreted and exuded by a froghopper.