I wouldn't worry about the purplish/reddish tinge. It's very common. Yep, that skin is coming off the lowest part of the mainstem. Those funny little bumps underneath will grow roots if you re-pot and plant more deeply, or add some dirt.
The experienced hands around here constantly counsel caution with nutes. Not til a month or so unless you're
sure you have a deficiency developing.
Whether it's too soon for nutes or not (I think it probably is) take a read thru this
little article, especially the last part about nutes and pH. That explains why your runoff suddenly went to 5.