Algae, we dislike it, but it is part of our hobby. It is also part of nature and is actually great and in many ways, beneficial for our ponds. Algae can assist you in maintaining healthy fish by taking up Nitrates and providing shade and a cloak from airborne predators, such as Herons. Algae are good indicators of the atrophic status of a water body, that is, the degree of pollution and nutrients in that water. A lake or pond dominated by green algae and diatoms is relatively "clean" oligotrophic water, whereas dominance by bloom-forming blue-green algae indicates a more polluted or eutrophic condition, typically caused by the troublesome Cyanobacteria or blue green algae.
Rather than treat your pond with caustic algaecide which is dangerous to Koi health, or create a UV (energy consuming) dependent pond (also dangerous), I will encourage you to consider using NUALGI PONDS for the remediation of green water. It is a natural approach which has proven to be extremely successful and it is so simple to use without consequence to Koi health. In fact, it creates living food for the Koi while it clears the pond!
IF you like to geek out on your pond water with your microscope, the algae cells on the charts below are considered to be the more prolific types found in closed recirculating ponds.
CLEAN WATER ALGAE
Green algae. These are thread-like, mat-forming algae; they do not attach to a substrate. The slender filaments have almost no branching but may produce short lateral projections called rhizoids. Cells are 2 to 4 times longer than wide, seen under magnification. These algae form large tangled coarse mats or skeins that float near the surface, held up by the oxygen they produce, or snagged by underwater objects. Generally found in shallow and hard water, Rhizoclonium grows in similar places as Cladophora, to which it is related.
POLLUTED WATER ALGAE
Green algae. Dark to bright green filamentous algae; free-floating and able to form cottony brown mats of pond scum at the surface. Found in shallow, quiet to slow-moving waters; common along shorelines and water fall areas. Individual threads are made up of cylindrical cells and rarely branch; they feel slimy due to a mucilaginous coating. These filaments grow parallel to each other, rather than crossed or twisted. When held in the hands the algal mass is slippery and untangled, able to be separated into long strings or strands. Colonies floating in colder, deeper water look cloudy or dispersed around the edges; in shallow, warmer waters the algae tend to form firmer, coarse growths and produce dense mats. Under magnification a green spiral is seen winding around the interior of each cell; this is a chloroplast in the shape of a helix.