How to move a layer using a slider control? I'm trying to create a mogrt file that has the option of moving a layer using a control (instead of just moving it on the timeline). 

Is there a way to create a slider control for example that allows to adjust some kind of "offset"?
Yes, in short. But you will need to learn Expression Controls. 

Search YouTube for "Expression Control Slider After Effects"

There are a lot of good tutorials which will walk you through the process.