Hey buddy, how's it going?
Hope you're doing well :)
It's a calcium deficiency, my friend.
Your cannabis plant may show signs of calcium deficiency if the pH at the roots is too low.
This happens because when the pH in your root zone is too acidic, your cannabis can't properly absorb calcium through its roots.
So, the first step is to make sure you have the correct pH at the next watering, and everything will go back to normal.
Good luck and hugs!