Regulus isn't too worried, yet, expecting he can fire off two stunners for the Andalites and perhaps an incarcerous to tie the other up before they're likely to have much idea he's there. The part that makes him frown is the suggesting of leaving the human conscious. He is not there to help them claim more hosts, if that's what she's considering.

Mikoto goes over her own world's spells, and decides they are too likely to draw attention for what is still primarily a stealth mission. She keeps one of her knives in hand just in case - though she's learned some spells from Regulus over the course of their research, including the stunner she expects he'll use, but if that doesn't work on whichever Andalite is nearest, she's just as comfortable with slitting their throat.