Dark spot on stem.

Any one know what this could be. Only on this one plant and doesn't feel soft or rotten. Plants are about 5-6 weeks old. Side note have done some lst(Bent main stem down a little bit)

Thanks from my first grow.

Edit: There is no yellowing of leaves/plant looks great. About 81 degrees in the tent with 55-57% RH, using FFOF, 3 GAL Smart Pots.