Alucard has used his mind hax twice, in addition to his fear hax, which is shown to include mentally taunting his target when he telepathically told Rip Van Winkle he was coming for her, all the time. So it's not exactly out of character to use it. If he finds himself being overwhelmed, and realized that Mercer is neither a vampire, nor a werewolf, nor a regenerator, the first two of which he'll probably know instantly, he likely would at very least use his telepathy and fear hax, and his mind hax would likely be used eventually. Walter and Anderson for example would have had resistance to it.