the green color is probably phytoplankton. it is a microscopic algae. if you do some researh here and on the web you should be able to learn as much about this tiny plant as you can stomach. It is possible that your aerator is stirring up the bottom "muck". this muck is rich in nutrients and can cause a plankton bloom. if you have a diffuser type aerator you may consider moving it off the bottom. there are some bacteria treatments available on the market that will use up some of the nutrients. check these out as well

I would not recommend using a copper based herbicide for plankton control, especially in such a small pond.