My guess is that he cross-threaded the puller into the arm. Steel on aluminum shouldn't gall but it will get good and tight when crossthreaded. I'm not sure why the OP is *****-footing around backing the puller out of the arm. It will come out though the crank arm threads are likely toast. The puller, being much harder, should be ok. As has been said, put it in a vice and go to town.