Until such time as the plants are more substantial, you don't have many options. Actually, none. Algae will use light and nutrients unless there are live plants that can use both faster. Once you have a good cover of floating plants (is that Water Sprite?) this should be controllable. In the interim, you could reduce the light photoperiod, but I would not go below five or six hours.
However, another issue is the light spectrum. And here there is some confusion. Cool white light is high in the blue but low in the red. Warm white is high in the red and low in the blue. The Kelvin number tells you where this sits. A K of 4100K is warm white, not cool, so that is confusing. The 6500K should be high in red, blue and green, and this is the best light for aquatic plants; photosynthesis is driven by red and blue (nd primarily red), with green adding more intensity. Sun at mid-day is somewhere around 6000K. If this light really is 4100K, it has the red but the blue and green would be less. When I experimented with many T8 tubes, I found anything below 5000K to be insufficient on its own. If you had two tubes, one could be 5000K and one 6500K, and this is just about ideal. The duration is still a factor of course, and this brighter light would likely still have algae issues, but once the floating plants are thriving, it should be much easier to keep problem algae in check.
I should also mention that not all 6500K (or whatever) light is the same; the phosphors and how they are manufactured plays into it, especially when it comes to intensity. Intensity and duration are not interchangeable if either is insufficient.