The opportunity cost means there is effectively no difference. A megawatt hour used to mine from a clean source means that a megawatt hour used for something else doesn't come from a clean energy source. Even if this project is the difference between building additional clean energy capacity or not, the resources for building that capacity have an opportunity cost—something that can't be built that would create value. Mining cryptocurrency doesn't create wealth any more than mining gold or printing money, it creates value in a zero sum manner, by discounting the value of all other currencies by a very tiny amount.