Lex had already considered the possibility. He had specifically trained in how to control his physique, though there wasn't exactly an instruction manual for how to control a near mythic physique that no one had ever heard of in history.
But he had a very good understanding of his body, which is why he was fairly certain that he could prevent it from being randomly triggered. What he had not anticipated was that using over a certain amount of his own blood to heal someone would not trigger a random outburst from his physique, but a targeted one.
But it was too late to stop now, so Lex kept using his blood to heal Moon. What he did do, though, was to summon the Butter Knife and use the divine energy to obscure any traces that the activation of his physique might leave behind.