Yea think of the Kelvin rating as the average color of your light, but if you look at the spectrograph of 2 different 6500k lights, they could look totally different. Everything that you are describing about your plants indicates you have more light, more specifically more PAR hitting your plants.
To answer your algae question, think of how different the tank looks with LEDS vs florescent. With florescent light is thrown in every which way, as this is the nature of the bulb, you have all these bulbs draped over the tank, with half of it being shined into reflectors. When you look at the tank, everything is illuminated, there are no shadows, which means for the algae-which doesn't need much light to grow-tons of suitable habitat. And since your plants were not red, and the internodal distances were increasing(stretching for light) I'm guessing the plants weren't too pleased either. Now contrast this with LED's, where you have a point source of very bright light, all coming from one direction, not being wasted on reflectors and scatter. You get shadows, and you get really bright light on the leaves, where it counts the most. The plants do better-which in general means less opportunity for the algae plus there are fewer places for algae to grow. In addition, you can ease back the photo period without slowing down plant growth by much, yet really taking the algae out at the knees.
Just my 2c