You might try moving the coral lower in the tank and see if it regains color. If it does then, of course, that is your answer.
I agree with you that I don't think it is nitrates as that will usually cause a coral to brown out not bleach out.
Super low nutrient tank can have problems maintaining color on corals but IME it is usually that they are a pale/pastel shade of their normal rich coloration. Yours appears to have lost all color. I have a former bright green hydrophora in my tank that suffers from the same thing - still grows - has great polyp extension (read shaggy bush) but only a pale brown/tan color. I am going to try and move it down and see if I can color it up again.
In low nutrient zeovit tanks an element that often gets depleted is potassium and that can also lead to very pale/ STN events and you have to suppliment with a potassium mix. You can buy a potassium test kit (they are a cursed test kit that is very difficult to read) and try that. I have this test kit and am actually coming to Calgary the week after next. PM me if you would like me to bring it down with me and you can borrow to see if this might be your problem.
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