Facebook App

I do not know if anyone can help me?

When I am Online I use FireFox to go on FaceBook.
And if I do not want anybody to know I am Online I click the Gear Icon at the Bottom Right.

It is ontop of the screen were I would type to somebody on FaceBook.

Then I just click Hide or whatever it says.

Then before I go off I do this again but turn it on.

But my Sister goes on FaceBook on her Android SmartPhone.
She has the FaceBook App.

My Sister was asking me when She is on her FaceBook App how She can have everybody not see that She is On FaceBook?

Like I say I know how to do this Online but not in a SmartPhone App.

Can anybody help me?