Algae, like most aquatic organisms, responds directly to temperature. That, and the availability of nutrients and light, determines when and at what rate it will grow.
Algae only becomes a water quality issue when it becomes quite thick. This is known as an algae bloom or by many pondkeepers as 'pea soup'. It is only when this much algae dies does it become problematic. Bacteria recognize the dead algae as a food source and will multiply based on the amount of this dead algae. When the bacteria 'feed' on the algae, they consume considerable amounts of Oxygen. In severe cases, this can result in almost complete Oxygen depletion and a fish kill.
Because of this, the use of any algaecide or UV in such a situation only accelerates the problem requiring considerable aeration to replace the Oxygen being removed by the bacteria.
Green water is a natural occurrence in natural ponds every Spring. The increase in water temperature added to the existing nutrient level that has accumulated over the Winter triggers this annual phenomenon.
I would not be concerned about this noticeable level of planktonic algae as once the water additionally warms zooplankton will begin to multiply and feed off of this algae. Over time clearing the water without affecting the Oxygen level. Zooplankton, along with Phytoplankton (algae) are natural components of the diet of most fish. This is part of Nature's way of re-cycling and is a large part of the aquatic Carbon cycle.
I would also not be concerned about the fluctuating temperatures effect on your fish. They are quite adept at adapting to temperature change as long as it not a sudden large change. Since water temperature changes at a much slower rate than air temperature and also changes over a longer period of time there should be no adverse effects to your fish.
