White spots keep reoccurring and moving around

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I have a wonderful montipora digi about the size of a 12" plate...recently small sections turn from green to white overnight

Does anyone know what is going on

 

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Also...color has slowly morphed from all green to more purple...I think this is due to do change in light spectrum...which I changed aback a couple days ago...went to more "twilight" t5s now going back to mure super actinic...but white spots started about eight weeks ago...you can see yellow where some were before