++++Just my opinion but I wouldn't let the water drain so near to my plant. I would get that tube away from the stem. The roots are plenty big enough now not to need to have it so close to the plant.
I'd appreciate that, I should of posted it there in the first place but I was unsure where to post this.I'll see if we can get this moved to the inermiry or hydro, get you some more brains on it.
Those spots on the stems are common and nothing to worry about.Also the bottom yellow and scrub little stems should be removed because they wont amount to anything,so basically theres nothing wrong with those plants.
I'm still pretty new to hydro myself, on my second grow, but everything I've read says not to keep rock wool constantly soaked as it can cause root rot. The Current Culture RDWC system I'm running stresses that the rock wool should remain about 1/4 to 1/2" above the water level. The bubbling from the air stone will put out enough moisture to keep the rock wood moist but not soaked. By the looks of your lower stems, right where they come out of the rock wool, and given that you also have algae growing on top of the rock wool suggest to me that could be your problem.
Just my opinion but I wouldn't let the water drain so near to my plant. I would get that tube away from the stem. The roots are plenty big enough now not to need to have it so close to the plant.