Your CUC is consuming it, it's just way to much.
What level do you run N and P at?
Is there any Sunlight hitting the tank?
What lighting type and photoperiod?
Answers can be as simple as age, excess nutrients or too much/incorrect light.
Also, how much rock versus water volume are we looking at.
It seems the problem is only on 2 of 3 rocks, seems strange.
Help us understand.