Gmlive only updates events, not code and variables in events.
Livecoding for GameMaker: Studio / GameMaker Studio 2 · By
When you're modifying that x value so make the increment the square moves bigger and smaller, you are not saving so GMLive doesn't know the step event has updated, unless I'm missing something here. I can see the * next to the filename which is what makes me think this.
What you are trying to do should work just fine with GML.