You know, I'm pretty sure that's actually a KASU. It's occurred to me, too, that the Yeerks used the word "Animorph" without having any reason to know it.
Perhaps it happened at some point off-camera, though, after the Yeerks found out the Animorphs were humans. Maybe some controller yelled "Andalite!" out of force of habit, and somebody corrected him by saying, <No, Animorph.> I can totally picture Rachel saying something like that right before taking a swipe at a Hork-Bajir.