it looks like your arm is able to compress itself right?
you should be able to get away with actually making the shell part of the arm itself
if you reshape/reposition the arm so it fits perfectly to the actual shell then all you have to do is animate it popping out and then extending the arm to show the gaps in the shell that you have showing in the yellow circle

otherwise if you stick with what you have you'll have to key the visibility and get the timing just right or it'll look funny

i dont know if you can put visibility into a setdriven key but i assume you can