I think what your talking about is normal...the main stem has a skin like structure that will sluff off as it grows, mine always did
As far as the yellowing leaf tips? I think it may be a little hot on the nutes but I may be wrong...maybe you should flush them a little to make sure not to get too much salt build up.
One thing I forgot to mention, you should water them with plane water first before you give them fertilizer, so that the uptake of the fert will be less potent and more easily accepted by the plant...or something to that effect?
I always did this the day/night before I gave them their nutes.
These babies are still quite small so, we really need to be careful not to over fertilize them.
Sometimes it's hard to know when to start giving them nutes, just back off on them for now and wait til they start getting more light green before feeding again..ok?
When they are dark green, they are well fed and don't need nutes but when they start getting light green they're getting hungry and need feeding....that's how you can tell and part of "reading" your plants.
learning to watch for these tell-tell signs, is all part of the learning process...cool?