It looks like maybe a fungus combined with a lack of nutrients.
Grow big shouldn't be used alone. At this stage you should be using 2 tsp. of Grow Big along with 2+ tbsp. of Big Bloom.
Fox Farms is a three part nutrient solution. Which needs to be mixed according to the plants specific needs at each stage of life. Just using Grow Big will not produce desired results. You should also invest into a Ph meter for your tap water. Without correct Ph your plant will not be able to pull what it needs from the soil.