I’m not sure exactly if the issue you are having is to do with mouse / pen / touchpad interface, a lag with after effects or user error – however –
Select your layer (Number of layer on Keyboard numpad or just with mouse) and press M to reveal any masks, you can delete, select and unselect them within the timeline rather than the composition window. You can also do this by selecting your mask in the composition window too. Pressing M reveals the Mask. Pressing MM reveals its properties.
Press F2 to unselect everything if you need to access the Move tool. Press V on Keyboard to access move tool and G to access pen tool. Pressing Q cycles through the defined Mask Shapes (Rectangle, Circle, Star etc).
Definitely get used to using Keyboard shortcuts, they will save you hours of work on every project.
Cheers!
David Byrne
Animo Motion Graphics
Freelancer, UK