Yeah, but Yeerks are way too arrogant and angry when an Andalite is around. For one, if you were a Yeerk, you would not expect Elfangor to give you morphing power since they believe in Seerow's kindness.
Yeah, if the Yeerk was Visser One or someone of equal self control and slyness, the Animorphs would be pretty much blown. Otherwise, the Controller would hiss "Andalite", or not get scared or surprised.