It doesn't look like powdery mildew to me but it's hard to tell 100% from the picture. Wet a paper towel or cloth and wipe your leaves down. If it's powdery mildew it will wipe right off. If it doesn't wipe off then it's something else.
In the case that it is powdery mildew just wipe it off and keep the humidity dome off from now on. Serenade spray is good for killing powdery mildew but since your plants are still seedlings it's easier to just wipe it off and make sure it doesn't come back. You're going to end up cutting the infected growth off anyway when you do your pruning.
yea, i kinda do this, i have a spray bottle i bought, for like 3 bucks, has a stream/mist setting, i put it on mist, and spray them all down. if i see something wierd, i go wipe it off with a soft towel, gently. if its still there, then its necrosis due to bugs/chems. if its gone, then it was dirt/mildew.
how high is the humidity? if ya cant buy anything to get rid of it, cuz your strapped for cash, lose the dome, and leave the lights on longer, let the heat build up a bit.
Make sure to NEVER water/spray them within an hour of the lights turning off. try to do it in the morning, so you know any surface moisture has seeped into the dirt/leaves, or evaporated.