Something seems odd here...we have a tank that cycled in three weeks and that has coralline algae spreading in record time. It all seems impossibly fast to me. The CUC is still alive after being in the tank the whole time? If they were in there while cycling, i would think they would have died with ammonia and nitrite spikes. I'm kinda questioning if this is all as it sounds.
How did you cycle the tank? What are your parameters? How are you testing? What kind of light? Your LFS
should know exactly what it is if you do not.