Alge

From Koipedia

Alge in a pond is not a cause for alarm as some people think. While alge in the water can make it hard to see the fish, and string alge can cause problems with filters it is not necessarily a sign of a sick pond or a healthy one and it is a natural healthy snack for koi to eat. The best way for Koi pond owners to remove water born alge (the kind that makes the water look green and hard to see through) is with a UV or Ultraviolet water treatment system. The technology is similar to what is used in some home or commercial drinking water treatment systems, a Ultraviolet light is exposed to the water killing alge and most other things living in the water depending on the amount of the the water remains in front of the light.