My pond has green water and it creating lots of bubble for the last 10-15 days. Why it is happening?

May be it is pond scum.
The technical definition is a filamentous algae that forms mats upon the water’s surface. These algae usually form on the bottom or sides of a pond and become buoyant and float to the water surface because of the oxygen bubbles it has produced. Once at the surface, pond scum can quickly cover the pond’s surface, giving the pond a dirty appearance. Remember pond scum is NOT floating plants like duck weed, water meal, or planktonic algae (green water).

Why does a pond get pond scum? Typically the formation of pond scum occurs from a high level of nutrient build up in the pond; mix that with higher water temperatures and longer daylight hours of summer months and a pond scum bloom is bound to happen. A common issue with declining natural ponds or farm ponds is pond scum because of the high nutrient levels these ponds tend to accumulate. In backyard constructed ponds and water gardens pond scum will occur for the same reasons (high nutrient, light exposure, high temperature) but the cause is usually due to poor pond management (over-feeding, over-stocking, dirty filters, excess debris in pond); or poor filtration, which is a rampant problem in backyard ponds with the majority of constructed backyard ponds being under filtered. Depending on the type of pond you have and the particular circumstances around the pond scum problem, there are different approaches to control or eliminate a pond scum problem in your pond.

For the natural pond many choose to utilize chemicals to kill the pond scum. Chemical treatment is quick, easy, fairly effective, and not outrageously expensive. However, chemical treatment, in my opinion does not really address the issue; it just hides it until later when the same issue floats to the surface again. My preferred approach is to diagnose the reason, the source of why the pond scum is occurring and attack the source of the problem. This could be as simple as not putting grass clippings into the pond, or redirecting some of the groundwater run-off. Treatment could also mean a good cleanout of the pond via dredging or manual cleanout of debris and installing aeration, or some type of filtration for the pond. Sometimes even adding desirable nutrient loving aquatic plants to the pond will resolve the problem. Treatment approaches such as pond shading also help to reduce pond scum returning. The proactive of supplementing ponds with beneficial bacteria treatments, or barley straw (extract) treatments can help improve water quality tremendously. These types of approaches actually deal with the source of the problem and provide long term solutions to controlling pond scum.
 
Reference:
http://www.fullserviceaquatics.com