2018b supports Tooltips.
"Tooltips: Create custom tooltips for UI components in apps Set the Tooltip property on a UI component to display a tooltip when the user hovers the mouse over the component at run time. Tooltips display even when the components are disabled.
The Tooltip property is available for UI components in App Designer apps and in figures created with the uifigure function.
If you are creating an app using GUIDE or the figure function, use the Tooltip property instead of the TooltipString property on UIControl, Table, Tab, PushTool, and ToggleTool UI components. For details, see TooltipString property is not recommended."