What is this?(pics)

I checked on my plant the other day,picked off the offendig leaves, i checked the next day and i still had them on my babys, they are only 3wks old,thought it was a nute def. so i gave it a feed, today same problem.
It's yellow,lookes powdery but on the back of the leave looks perfect, its leaves were turning over the other day but feeding reversed this, thought it would help the plant, i have rubbed it but it doesnt move, dont know whats going on,all help greatly received,
600W HPS ,dont know ph , hydro,temp, this has been my 3rd attempt and theres been problems every grow,indoors closet,lights raised.peace.
P3dD1977

check ph

Dont have a ph meter till wednesday as i need to get a thermometer also, thanks again will post ph levels wednesday or thursday,

A ph meter won't save you here, as its but common nute burn, assuming the soil is fresh very few plants will indicate (N) def in under 30 days of veg, use common sense, and wash that excess from the rootzone, allow to drain overnite to my blog:

https://www.rollitup.org/Journal/Entry/flush-baby-flush.27513/

plus:

http://forum.grasscity.com/blog/1093/entry-9710-three-essential-tools/

should you need ph shit, leap to your local drugstore and bleed $5.00 on pool strips

ps nice shoes!