I'm cycling my new planted tank I would like some algae to grow on the rocks so while cycling could I stick this light on for a few weeks
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I would advise against that. The sheer number of nutrients that are present will be hard to combat when your cycle ends. I wouldn't turn on the lights at all.
As for growing algae on the rocks. Place the rocks in a container with just enough water to submerge them and then place the container on a window seal or somewhere where they'll be exposed to an ample amount of light. And algae will form that way.