Final answer:
The control to show a customized help message for form elements is a ToolTip, which appears when a user hovers over the control.
Step-by-step explanation:
The control used to display a short, customized help message for individual controls on a form in user interfaces is called a ToolTip. This is a small pop-up window that provides brief information about the function or use of a control when a user hovers over it with the cursor. For example, in a graphical user interface on a Windows form, a ToolTip might appear when a user hovers over a toolbar button, providing information on what action the button performs.
ToolTips are essential for enhancing user experience by offering immediate access to additional information without cluttering the interface. Developers can customize the text displayed as well as the duration and appearance of the ToolTip.