Hiya
I guess you have reduced your feeding of the clam food as you say they are off colour. Also the high Nitrates will effect the clams colour they hate any nitrate.
Find the nitrate cause.
Increase the Min-S to 2ml a day and let me know how often you are feeding ultra clam
You are now in the process of fine tuning your system.
Add 2ml of min-s as of today