first time grow. please help!

on one of my plants i have a white powder like substance on the leaves. on my other plant, basically the whole thing is yellow/green and has serious burn marks on it. let me know what you think it is and what i can do to cure it.

I would check the following and report back.
1. Soil PH.
2. Room temps day and night.
3. Humidity.
4. Co2 if using.
5. Nute schedule.
6. Frequency of watering and how much, soil moisture check.
7. Did you add calcium or magnesium? When how much.
8. Lights and type, distance from plant.

i have 6 T-12 lightbulbs on a 18-6 light schedule. lights are about 4 inches from the plant. i did not use Co2. water about every other day. ill get back to you with the rest of details

do you have any idea on what it could be?