Problems w Growing - Beginner - Mold? Burn?

I'm a new grower and have been dealing w a problem on one of my plants.
Anyone have any idea what's wrong?

fill out the trouble shooting form. one question though, do you use bottled or RO water with out calmag? if you are then thats your problem but if not fill out that form with a whole plant pic. just one leaf isnt going to be an accurate diagnosis