As for Alex, the moment he felt the pressure of Caius' aura, he decided to make a gamble.
If Caius was a Variant class, then he wouldn't have resorted to revealing his true level of power. But now that he had revealed himself to be an A- rank Mutant class, this gave him some level of confidence to fight him. If he was any stronger then he would have just brought Brontes out of his slumber in the taming space under the pretense of giving in, only to then have Brontes attack them.
But now he had the confidence to do the dirty work himself. Well, not 100% confidence… but it was as good as it got for the time being. This was the perfect chance to test out his new level of power.