Isnt it cyano
that reacts to the light like this. I maybe off on this one, but that sounds right. Mabye someone can clear it up.
Another possibility is "light shock." Just like you can bleach corals when you upgrade from p/c lighting to m/h unless you acclimate the tank to it, you may have starved the bad algae then fed it a huge meal (so to speak). No light for several days...then straight back up to 7hrs a day.
Also..what kind of bulbs do you use? Im assuming h/o was suppose to be N/O lighting. It sounds like you have been around long enough to know not to use shop lights because of the spectrum problems.
Have you considered putting a fuge on the tank to suck up the excess nutrients in the tank?
The one parameter you didnt mention was PO4
Sorry for all the different options..just trying to cover all the possibilities.