I agree looks like the  beginnings of nute burn. Usually when I have nute burn it happens over night so I had not seen it in process. Interesting.  I see its super soil and know less than nothing about that but a good flush never hurt any plants that I know of, so I would start there.  The soil growers on here can probably tell you how long to flush for and if its a good idea or not. 

That looks like its leaning towards Sativa in the picture and they can be super nutrient sensitive