Are the roots slimey at all?? If not it is most likely nute stain. I know you lis FF as nutes but what all do you have in your mix?? I use GH line and as soon as I add Florishious Plus it does the same thing. If it is stained a touch of H2O2 will clean them right up but dont overdo as H2O2 can also kill to much beneficial bateria also :thumbsup: