I just figured it out thanks to AI.
A white circle appears when pressing Ctrl in a game because a feature in your operating system or an installed program, like Microsoft Power Toys, has been activated to highlight the mouse cursor's location. To disable this, you can either go to your Windows Mouse settings and uncheck "Show location of pointer when I press the Ctrl key" or turn off the "Find my mouse" feature in Power Toys if you have it installed.