That stem is a bit strange but I'm too new to this to know exactly what it means. In older plants I thought it meant nutrient lock-out but that wouldn't really apply to a seedling.

I don't think it started in an ideal situation with you not knowing what's gotten into that pot but I am not sure changing soil now would be good either, those are delicate little roots growing under there.

If you do that I would try to contain the bits closest around the plant (and beneath it) to keep from stressing the fragile little roots. Just replace the outer bits of soil. (If that makes sense)

I don't know mate. MJ is a fairly tough plant, it might be just fine as it is.