Help, just noticed fan leaf problem

I just checked on my baby and while i was adjusting the wires for her lst'ing i noticed that there was one set of fan leaves that were looking really yellow compared to the rest and even have some browning going on, i thought maybe magnesium, but the stem for this fan set is green still so i dont know what to make of it, rest of her is green as can be and growing great! help please and thank you ahead of time heres a picture showing it.

 

Yes thats it exactly so should i add more phosphorous?