Someone please help save my plants!! Moldy-like substance forming on one....

....I just went to feed my plants and noticed a powdery, white substance on the leaves of my Moby Dick plant!!! My first reaction was that this is mold, but im still a noob and havent had any problems thus far. I have 6 females (different strains) which I just put into flowering and also have one plant in the middle (lowryder) that is almost done flowering. They are under a 600 HPS and am using ocean forest as a medium and full line of FoxFarm Nutes. This substance comes off easily when I wipe with my fingers. Heres a Pic:




A couple questions:

What in the world is this and what could have caused this?

Any help would be greatly appreciated

Looks like PM, same problem I'm having its a real bitch and highly contagious. If they don't all have it I recommend you isolate the problem plant for treatment asap!