HELP! White spots/leaf damage on tomato seedlings

[DannyD]

Hi, I need help diagnosing an issue affecting some Tappy's Heritage tomato seedlings I transplanted on 4/3. A few days ago, I noticed white spots and some leaf damage on some of their leaves.

Planted these from seed around the start of March in an organic seedling mix from Scotts or one of the other big soil/fertilizer producers. They've been on a windowsill that gets 6-8 hours of direct sunlight and good indirect light the rest of the day. Our apartment tends to be pretty dry.

Was thinking this might be a case of powdery mildew...? But not sure. I have little experience diagnosing plant issues and am just going off the pics in troubleshooting guides some of the university extensions put out.