First indoor grow...could use some advice please!!

I am growing aeroponically in a smaller cab for first time. Growing in rockwool w/ hydroton. Been using a 150w hps light and gen hydro maxigrow nutes (high in N). Temp has been 80F during the day and 70-75F at night. Have kept water at 5.5-6.5 ph and 350-450ppm In 3rd week of veg growth. Have six plants going and a few of their stems have developed a white salt like substance at their base. I made a few adjustments and all seemed well. Then i awoke to find the biggest plant (sex undetermined thus far) which was approx. 14-16 inches tall had fallen over. When I tried to straighten it and stake it I snapped it completely in half near the base of the plant. Well I took cuttings from this plant for clones and discarded the rest of it. So my questions are.....

1.) How do I remove the net cup with the remainder of the plant that broke? Stem, roots system, etc. Its roots are entangled with the other plants roots and I am scared I will do them damage if I try to cut too much away. can i just cut at rootball and leave whats left laying in the bottom of res tank?

2.) Would anyone know what the salt like substance is on the stems of the other plants? Is this why the biggest fell over? Is it a mold or fungus that will take out whole crop? The others still seem strong and healthy and leaves all look good....just wanna solve this before it gets out of hand!!! This is my first post and look forward to getting some great advice. I will post pics asap so it will be easier to diagnose.......Thank you all so much!!!

So i just pulled the net cup out as far as i could and delicately cut the roots away as far down as I could without getting into the mass of roots in the res and let the rest fall to the bottom. I wiped stems off. Substance was alot like salt so hoping that its just a salt build up and not a mold or fungus. Does it sound like i made the right decision? Wasn't getting any replies....