What are you using as a growing media?
What type of water are you using?
What is the pH of the water before you mix up the
nutrients?
How much Calmag are you using?
What are you using to adjust your pH?
If there is algae growing in the
nutrient solution you can block light to the reservoir by painting the outside black, or putting a dark blanket or box over it. It looks like you are using the black sewer tubing for the troughs. That is light proof so if you are getting the same growth inside the tubing that wouldn't be algae. My guess is it might be a byproduct of the nutrients. Such as residue from the mineral elements, or growth of the microorganisms in organic nutrients. I'm have never used those nutrients but you can find out by e-mailing General Hydroponics Tech support and asking them since it's there product.