I took the filter screen in my photo above to a water testing lab today and talked to the manager. He could not tell what it was by looking at it and said of the few things it *could* possibly be is iron bacteria (not harmful to humans) or algae and the only way to know is by lab testing. Problem is, it needs to be fresh and still wet with the bacteria still in it. I took a few sampling bottles with me and will keep on eye on it this coming season and see if it shows up at any particular CG then I will take a sample into the lab by the next day. It would be interesting to know if it's coming from the CG we regularly use. The lab manager said if I can figure out which CG it's coming from to notify them so they can do something about it because it definitely shouldn't be like that.