A mask has been created:
The shortcut Alt+M will clear it. Is there a button in the user interface? Or a menu entry?
AFAIK there is no proper button, the way to clear the mask without a keyboard shortcut is the menu entry Hide/Mask>Clear Mask.
You can clear it using Alt+M
[You have a button under Hide/Mask]
But its shortcut is ALT+M
On the bottom there's a list of Keyboard Shortcuts