Slimy stuff in my water reservoir

It is bacterial growth, maybe mixed in with some precipitated nutrients. If you are not running a sterile nutrient reservoir I would add in hydro grade hydrogen peroxide (30% 2.5ml/gallon, be careful about chemical burns) from your local grow store, or use a product like hydroguard. If your res isn't smelly and your ph is not swinging wildly, it's probably not too serious. Cleaning out the whole reservoir once a week and getting it off the accessories like the airstone and tubing can be helpful. If I try pushing it to 2 weeks between cleanings I get the same thing, even with the initial hydrogen peroxide dosing. The plant is otherwise looking pretty healthy, happy growing! 🌱👍