I know Birdsnest is can be happy with much higher levels than that but I wonder how bright your lights are? If it was not getting enough phosphate to grow zooxanthellae AND it's under lower light, like 50 - 100 PAR, it may have browned because it's grown more zooxanthellae. What you were seeing before was colors induced by being partially bleached from lack of food.
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"Our crystal clear aquaria come nowhere close to the nutrient loads that swirl around natural reefs" Charles Delbeek
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