The very first thing Alex did when the situation seemed to get out of hand was ask Memory to absorb all of the energy within it at the moment. So long as Memory could consume it all, there would be nothing there for him to worry about.
Memory responded to the request immediately and began absorbing the energy within it at that very time. However, even then, it was too slow and the energy buildup within the cauldron was hitting its limit.
If Alex didn't do something else, Memory would explode. But, if he let the energy go out here, it would still be disastrous nonetheless. It would be no less severe than getting attacked by an Immortal in the Immortal Spirit realm.
Alex may survive that, but he would definitely be wounded. And worse of all, Memory would be damaged. He couldn't afford that.