The expansion of the universe is about galaxies moving away from each other. There is no general increase of distance between stars within any one galaxy. There is nothing at all going on at even smaller scales, so far as cosmic expansion. That is the standard way of thinking in physics.
If atoms did change over time in a way related to cosmic expansion, we'd have trouble with defining units of time and distance. For the last couple decades, the second is defined certain vibrations of pure Cesium 133, and the fundamental space-time constant, speed of light, is a stated fixed number. If atoms did something odd like slow down slightly over millions of years - how would we know? So any effort to uncover novel new phenomena normally concentrate on pure numbers (without units) such as electron/proton mass ratio and the fine structure constant.
This gradual change is pure theoretical speculation. So far, unless I missed something, there's very little published experimental work on observing any such phenomena. (I didn't say 'none', but 'very little'...)