Bill - there are numerous types of algae, ranging from simple (phytoplankton) to advanced forms (chara).
I suspect that you're observing filamentous algae (stringy-slimmy mats), which can grow on the pond's bottom wherever sunlight is able to penetrate with sufficient intensity. As it matures, gas bubbles become trapped in its mass of filaments, prompting the mass to break free from the bottom and float to the surface - where it migrates wherever the wind takes it.
The purpose of a pond fertilization program is to promote a phytoplankton "bloom", which benefits the pond's food chain - but ALSO helps to reduce sunlight penetration, thereby reducing the amount of suitable habitat for filamentous algae growth.
Should you fertilize? Guess it depends upon the type of algae that presently occupies your pond.

Kelly Duffie
Cypress, TX