Certified GKH Gardening Expert
Unfortunately, your photo did not come through. I am unable to identify the growth in question.
Usually, white mycelium is safe and healthy. It can be a natural part of the root zone. When growing, hydroponically, it will be necessary to check the pH, and nutrient content of the water, first. This is the most common issue with any hydroponic set up. If your pH is perfect, your temperature is perfect, and your EC is perfect, THEN you can begin to diagnose problems.
My best guess is a nutrient deficiency unrelated to the growth, which could be attributed to a pH imbalance. This should be checked, and corrected, daily. I am unable to confirm that without visual evidence, however.