Why is my pond green
This appears to be an algal bloom. Algae, and other phytoplanktons, are naturally occuring organisms in almost all aquatic systems that capture the suns energy and CO2 to grow (just like plants).
In most aquatic systems alga growth is limited by not by sun or CO2, but by micronutrients they need such as Nitrogen and most commonly phosphorus. These nutrients are found in fertilizers and dead or decomposing materials such as leaves, grass clippings, etc.
I can not say what for sure caused this bloom in your pond, but if you or a nearby neighbor has used a fertilizer on their lawn or garden, particularly one containing phosphorus, then it is likely that rain caused some of that fertilizer to end up in your pond triggering the population explosion of alga that you are now seeing.
To prevent algal blooms in the future limit the use of synthetic fertilizers, and apply sparingly. If fertilizer is not the culprit, perhaps an excess of dead plant material found it's way into the pond. Again, reduce that input of this material and blooms will be less likely in the future.
You can also help reduce nutrient loading into your pond by leaving a vegetative buffer between your pond and your lawn or house area. Do not mow up to the edge of the pond, and leave a 2-5 foot buffer of plants on the edge of the pond that will help absorb nutrients before they enter the pond.
There are some materials, such as phos-lock, that will bind the existing phosphorus in your pond and prevent further growth. But these are just a temporary solution if you continue to have phosphorus inputs. Similarly algalcides (chemicals that kill algae) can be used to kill the algae, but this will not solve the overal problem of nutrient input.
Overtime your algae will die off, and this will decompose and use up all the oxygen in the pond, causing dead zones. You may want to consider aerating your pond if you have fish or other things you want to keep alive in there in the near future. This will help improve the overall fish habitat of your pond and help prevent smells from anaerobic digestion of dead stuff.
Hope that helps! Feel free to reach out with more questions if you have them.