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It is very hard to say without much context to the photo. I would try things like bleach, vinegar, baking soda, household cleaners, or pressure washing. These will all be good ways to remove stains from surfaces.

Your fears would be correct. There are plenty of alternatives to this dangerous practice. This article will help:

https://www.gardeningknowhow.com/plant-problems/pests/pesticides/mothballs-in-gardens.htm

I believe you are referring to Bishop's Cap or Mitella diphylla. There's no way to tell from here whether your plant has crossed over, but I think the best way to know is to cut it back, feed and water it, and see if it takes off. If there is any part that's still green it may have a chance. Good luck with this. Here's an article about Bishop's Cap:

https://www.gardeningknowhow.com/ornamental/groundcover/bishops-cap/growing-bishops-cap-plants.htm

Have you tried shortening the amount of time you leave them outside in the beginning? Do you bring them in at night the first few days? Try changing your routine a bit and see if that helps.

https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/how-to-harden-off-your-seedlings.htm

I am uncertain of the exact question. Please include details and photos if possible.