Read full tutorial here https://fluttermaster.com/transform-floatingactionbutton-into-a-cute-little-ghost/
You can customize with any emojis.
Cute ghost
floatingActionButton: FloatingActionButton(
onPressed: () => print('Clicked'),
foregroundColor: Colors.white,
backgroundColor: Colors.black,
child: Text(
'👻',
style: TextStyle(fontSize: 28.0),
),
),
floatingActionButtonLocation: FloatingActionButtonLocation.centerFloat,