Are the girls in with a male? If so, it is not uncommon to find bite marks from breeding. If you have more than one female housed together, sometimes they will squabble over food or territory. When they shed, the skin will adhere to the bite. The best way to remove it is what Mandi advised, unless it looks like a scab underneath and then it should just be left alone to heal.
If you see frequent bites, you may need to separate the geckos to prevent injury.