Is the white substance that forms between the toes considered a barrier that prevents water from reaching the skin during ablution, keeping in mind that it regenerates quickly and is removed by rubbing, and ablution is invalidated when it is removed during washing?

This white substance that disappears when rubbed off is dirt originating from the body, and it does not constitute a barrier (hail) for wudu or ghusl. The questioner should ignore the obsessive thoughts that have taken hold of her and wash her foot normally without going to great lengths to remove that substance.