That sounds like algae or fungus of some kind! I would call an aquarium shop and ask them how to treat the water. You surely don't want your koi to die on you!

This color means algae is present and you need either an algacide cleaner or an algae eating fish.

It is algae growth (tiny green floating plants, microscopic tiny). There are filters on the market which require filters to be changed. However there is a filter you can build which is biological and never has to be replaced: Basically it is like taking a plastic trash drum, putting a drainage pipe down the center, say a 4" white drainage pipe which has holes drilled regularly down its length. Around that you throw in foam, not packing foam, foam like in cushions which is full of microscopic holes and will sink when wet. Fill the can with water, run a pump which pulls the water from the fish pond, pumps it down the center hole and the can drains back into the pond. If you wait for about 6 months Mom Nature will take over and bacteria will grow in the cells of the foam which will eat the algae and also hold on to ammonias and release good things for the water. These bacteria can be bought if you are impatient, they are called Anaerobic bacteria. Those are placed in the filter tank NOT the pond. They will colonize the foam and will capture the algae and other things which they consider to be food. They prefer low to no oxygen since they produce oxygen as a waste product. ----- I did something similar, instead of using trash can I built an upper pond which is about 5 feet in diameter, 3 feet deep in the center and has a ledge around the interior wall that allows for the placement of potted water plants. Those "hide" the upper pond and make everything look really nice. My pump sits across the pond, pumps the water from there, up into the upper pond which then flows back to the koi pond via a water fall. I waited patiently it took about 6 months for the water to completely clear up. However since then the water has been fresh and clean, even from suspended dust. I took it a step further and placed the pump in a barrel that has one end lower - what happens is that fallen leaves/debris is skimmed off the surface, since the water flows to the pump and the debris collects in a wire basket I made. I made my own because the prices are way to expensive and I couldn't find a model/make of filtration system that fitted my needs.