Sps geting a green tinge at base

[zahira]

I have a solid blue acro which has lately started to get a green tinge at the base. I think it is receeding but not sure why the green tinge. Is it lack of a trace element or something else. My Ca is 390, dKh is 7.7, Mg is 1280, phosphates 0.03 and nitrates 50 (have always been high). The acro has been in the tank for the last two months.