Looks like root rot to me. (overwatering)

If you aren't measuring your nutes, you'll never be able to adjust them to suit the plant's needs. If it's Miracle Grow All Purpose, 1/2 tsp:gallon per week. Let the plant's soil dry between waterings.

If you really want to grow cannabis, start reading. If you just want to throw seeds in a pot expecting them to care for themselves...perhaps a chia pet would suffice. :thumbsup: