Hey buddy it appears you may have a couple issues here.
1st looks like you may have poor drainage soil. If it retains water to long it can cause root issues and that shows in the plant above soil with low stem strength.
2nd. It does appear you have the start of a cal/mag issue. This could be from improper PH water source or if you are using RO water. If you have not tested your tap source I would use RO water and add my Cal/Mag to what I need. I shoot for 250ppm then add any nutrients after cal/mag.
Cheers my friend and happy growing!