Green digi turning brown

I recently bought good sized piece of green digi from a member of a local reef club and had been doing well for a few months. I had fraged a couple of pieces of to keep from shading a different piece of coral on the same rock. The frag is doing great and has grown a little but the mother colony is turning brown and not looking great. Here are a few pics...

The whole rock



The frag



The frag and at about the same height in the tank and in about the same flow. I have 216w of t5s and new bulbs. All tests are within normal limits except the calcium in a little low. All of my other sps and lps and going great, any Ideas on what it going on. Thanks as always Rob.

 

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What are you kH levels? Also what is your nutrient levels, NO3 and PO4?

So the mother is turning brown but not the frag?

 

I have had a purple monti digitata turn more maroon/brownish because I didn't have it under enough light. This however looks like it is cased by too much nutrients or something else, can't say for sure without knowing your parameters.

 

No3 ~ 15ppm, NO2 ~ 0ppm, KH ~ 200ppm, pH ~ 8.1, Yeah the frag is doing very well has grown and looking the same color. The mother is just a little closer to the lights, about 1/2 inch, and in about the same amount of flow. There are 2 other sps on the same rock as you can see in the first pic and they look great. Not sure what to do. Rob

 

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