After trying a few passwords that Dumbledore had used before, and finding that they had no effect on the stone monster in front of him, Harry gave up the idea of continuing to try. The three of them realized that it was Snape who changed the password of the stone monster. Extra complicated, that almost meant Snape became Headmaster of Hogwarts. Only the principal is qualified to do that. They had to stand in front of the stone monster and wait patiently for Albert to come and pick them up.