Algae matters because it can obscure your view of the fish and also, string algae can choke a filter.

Good Reasons Algae Matters

1. Well Algae seems to have some sort of healing effect on fish, whether that is actually a function of “A good home for algae is a good home for fish” or the algae actually provide some sort of consoling cover, or even a healthy nutrient base….Unknown, all good theories. I am sure the pundits know for sure, based on zero research; but careful reading of Grainger’s Electronics and Machine Parts Catalog. (That’s an obscure joke about how awful the extrapolation with some people can be)
2. Algae oxygenates.
3. Algae consumes excess nitrate and phosphorus. As some people know, high Nitrate levels are eventually immune suppressive if only as a chronic stressor but equally likely, as a cause of perpetual vasodilation. (injected, veiny fins)
4. String algae produces what are known as “plant agglutinins” which are ‘sticky proteins’ which bind organic macromolecules and bacteria into large pieces to settle out, ostensibly dropping the particles to a position closer to the plants to be used by their absorptive structures. (Yes, plants have “arrows” to collect their “prey”

Why String Algae Matters

1. Koi will eat String algae except if better food is around they might leave enough to be ugly.
2. String algae breaks off and spreads and it can glut a pond
3. String algae breaks off and goes to the filter pump and other filtration structures (the mechanical elements) and pretty much stops it.
4. If the pH drops a lot, the string algae will die and being a fairly massive plant mass, it will create surplus decay products (nitrogen) and will contribute massively to water quality deterioration even after the pH is corrected.

Why Greenwater Matters

Same thing;

1. If the pH drops a lot, the string algae will die and being a fairly massive plant mass, it will create surplus decay products (nitrogen) and will contribute massively to water quality deterioration even after the pH is corrected.
2. Greenwater is not-nice because you got the pond to watch your fish, but noo-o-o-o-ooo not with greenwater.
3. Worse, greenwater stops you from being able to monitor the occurrence of disease, but also to monitor improvement of diseases you already know about.
4. A large dose of certain medicines can kill alot of greenwater and again, that die off can contribute massive amounts of nitrogen waste as well as consuming enormous amounts of carbon dioxide and oxygen consumption.