it just looks like algae to me. are you using RO water? whats the water temperature? you should consider a product like Hygrozyme which helps clean your roots and conditions the water and promotes new root growth. It can be used in conjunction with a little h2o2. but i expect your water is to warm and your getting algae. if you wait to long, it can end up as rot. do you change your reservoir out ever or just top off?