The parts of the black mollys that haven't got the coating are perfectly normal looking. The one which has it worse has a very clamped tail (been like that for a week or so) but is eating well and reasonably active considering.
This coating is paler and slightly greyer than fungus and doesn't fuzz out at all. It's like a sheen on it, like it's been dipped in something, it's not in patches - it starts about halfway down the body and covers the entire back half totally.