I'm not a hydro expert by ANY stretch, but a mss of white roots is GOOD. In fact, you should have a bit more root mass. That plant looks just a touch on the droopy side but otherwise healthy to me! Crap on top of the rockwool looks like algae... you can put some perlite over it to keep it from growing, but I'm not sure how much of a problem it really is. As I said, I don't really do hydro.