white stains on inner leaves

Hey guys. I'm 3 weeks into the veg cycle. Using 23w CFL equivalent to 100w, 3-4 inches away from four plants in 1.5l pots mixed with peat, perlite and light cow manure. Plants are growing well except for one. Since they're young, I have been feedingvthem lightly every fourth day and water every 4th day letting the liquid drain from the holes in the pots. I have used mulch to retain moisture.

Onevplant is developing what looks like white/green patches coming from the middle of the growing area. Anyone knowvwhat this is?? I have em on a 20 hour on and 4 off cycle with 25 degree Celsius temps and 47-55% humidity on average.