Certified GKH Gardening Expert
This sounds like powdery mildew. This can happen is the area remains too moist for too long. I would suggest supplementing light with at least 100 watts per plant. This will help dry things up a bit.
You can also use neem oil to help you get rid of the issue. This should be used out of the bright light, so that you do not burn them.
Here is an article that can help: https://www.gardeningknowhow.com/edible/vegetables/tomato/growing-indoor-tomatoes.htm