Don't stick them under the lights with others in veg just yet. It will spread if your environment favors PM growth. Deal with the PM first. If your strapped for cash use 1 tbs Baking soda in 1gal of water. Maybe a few drops of liquid castile soap. (Dr. Bronner is good)..
PM also can not grow in a high pH range so if you raise the pH of soil or wipe down the leaves with a high pH solution, you will kill the fungus. When a problem like this arises, sometimes the best bet is to control it, not try to eliminate it since you can damage foliage by spraying with this and that and god know what.
Also, yes, PM is from too much humidity and not enough airflow, but it can not survive in water. It sounds crazy but water will kill this fungus. You can wipe down the leaves everyday in the morning with plain tap water (better if your tap is a high pH) and this will control it, and possibly eliminate it. It's time consuming and you must pay attention to detail, but it can be done.
To prevent further infection keep you night temps on the warm side and provide ALOT of circulation.
Or Get SNS 244 fungicide spray( I advise to use this because it's natural and pretty safe to use). That stuff kill the spores. You can spray the leaves and spray all over the Ambient area of the plants. SNS fungicide is organic and will inhibit spores from spreading to other plants.
If all else fails, Toss the problematic clone, Weed will never be eradicated so it's not like it's the last clone you will ever get your hands on. Patience is a virtue.