Cyano dying back under lights?

[AquaLogic]

I have some red cyano that grows mostly on the sand, and I was doing a period of lights out to try to control it. I'm seeing odd behavior. The Cyano is actually growing strong at night with lights out, or under low blue light. When I turn my light (Kessil a500x) up brighter and whiter, the cyano dies back. I've never seen anything like this before. I'm managing the problem alright for the moment, but I found this very curious. Has anyone else see cyano behave this way?

 

[Weaverjay101]

Following along to find out more.

 

[DED65]

Do you have any pictures?

 

[AquaLogic]

Well, I think I figured it out. It seems I am getting diatoms in the sand bed as well, but they hadn't shown up enough yet to see. So, when the lights come on the diatoms are outcompeting the cyano, and when the lights go off the cyano out competes the diatoms. This is my theory at any rate.