There is no central Al Mustafa account.
Before deleting
You can export reading/Khatmah data from the home page or Quran Khatmah drawer.
Website data
Browser storage may contain reading position, bookmark, theme, Web Khatmah, cached prayer location/times and a limited recent Quran-page cache. Clear site data for al-mustafa.pages.dev to remove it.
Android app data
Clear Al Mustafa app data in Android settings or uninstall the app.