Yellow color of your aquarium water suggests that you have a buildup of phenols, phenolic compounds, carbohydrates from secretion of algae etc. This can be detected by taking a water sample from your aquarium and placing it in a white dish. At higher levels they can be harmful to your tank. You can read more on reef dreams
Phenol and phenolic compounds inhibit the growth of diatoms and therefore render Nualgi ineffective!
To remove these compounds from your water, you will need to perform a series of rapid water changes to dilute them out of your water and ensure your filter media is clean, your skimmer is working well, is of sufficient capacity relative to your tank size, and you are changing your carbon filter in a timely manner. Once the water color is determined to be normal, you can begin the Nualgi dosing.
In planted aquariums the phenolic compounds are absorbed by the vacuoles in the leaf and stored safely; hence we do not see this issue in planted aquariums.