Layer movement inside mogrts

Hi There

Im learning how to create mogrt and learning expressions and i was wondering if anyone could give me a bit of advice on the below?

i have a rectangle main body with a smaller rectangle inside, i need to give the ability for the smaller rectangle to move from one side to the other when the user wants it to and then back to its original position when they want it to. 

im very very new to expressions and would love to know if there is a way to do this? i have seen ways of doing similar with texts but was wondering if there is a way to do it with shape layers, pre-comps etc

thank you

Kind regards

Hugo Campos