First, instead of creating your slider with left and right content, why don't you directly create the slider without any of that from the MaterialUI menu?
Just select MaterialUI -> Slider -> Continuous -> Simple.
Then, if you don't want the popup. Just select your slider, and in the Inspector view, look for the MaterialInputField component. Then just untoggle the "Has Popup" field and you should be ready :)
We also made a video about sliders right here: https://www.youtube.com/watch?v=G4osdlSrLJk
Please try this and tell us if you still have issues!
Have a great day.