It's a bacteria bloom which means you overdosed it, also leads to low 02 in the water which isn't good I hope you have a skimmer running. For NOPOX it's the same as vodka dosing so you have to ramp up slowly, there's an article in Advanced Aquarist about vodka dosing if you look it up.
Tell us more about the tank, size, age, what's in it. Also I would second the suggestion that you read up on carbon dosing. A lot of people just do it to be doing it and don't know why they are doing it or if they even need to.
OD!!!
Stop using it for a few days, it's should go away and when it does just cut back on the dose.
I used before and I work great just do it slow
ANYTHING THAT YOU ADD TO YOUR REEF DO IT SLOW NO NEED TO RUSH !!��
All here correct.....start small dosing and increase a couple ml each few weeks. My volume recommended 12-14 ml a day. I have only run 2-5 ml. First time I rand 8-10 it could end badly....two separate systems now with the low dosage. Do a water change to reduce nopox and keep O2 ok.....