Its pretty normal to have on the main stem when they are young, almost like they are shedding a little skin. Next time get some good soil, you have to be extremely careful with MG, you can burn the shit out of them if you over water even a little because of the time release nutes. Yours look a little burned, and hang dog.
It's normal, imagine a Snake shedding it skin when it outgrows it. Your plant is doing the same thing, that first stem that your seedling had it outgrew, so its just shedding the extra layer.