I've had this GSP in my tank for about two months (previous owner had it for 2 years). About a month ago, a small portion on the lower left starts to fade in color and became cream/slight pink when opened up. It has since extend to a larger patch as shown in the photo on the left. Is this an indication of water quality? My other corals such as duncan, acan, sps (acro, valida, cali blue tort, etc), frogspawn, hammer, all have thrive. Nitrate is always 5-10. nitrite 0. Alk 9.2, Ca 490, Mg 1500+. I do about 30% WC every 3 weeks. Any idea? I know nitrate is not the best but it's not too bad either...