Weird, I am also first timer. But I've kinda been doing things I shouldn't. I read and read on anything I could find on how to grow and I've yet to see that. It looks like it could be white mildew starting to form. But if it don't wipe off it may be some kind of deficiency is it on all of the canopy or just in that leaf. If it's just the one leaf I wouldn't sweat it to much, but if it's all over check the humidity in the room as well as the temp. If it gets to cool and humidity is high white mildew can start to show. easy fix though hydrogen peroxide and water, or if your like me you can get what you need in most grow shops. I started with 10 for my first try and the only time I saw Nute burn the tips turn yellow and acually burn. And I got frogged with the whole mackerel (I am using fox farm) and gave a couple nitro toxicity but they were young and pretty resilient. Good luck!