Keying out greenscreen creative cloud express

Yeah, it's a tough one for that.  The "remove background" might be Adobe Sensei powered with the aid of Adobe Photoshop. Could be a good start, but you might be better off starting in Photoshop then, making your asset with some appropriate tools.

Alternatively, you could create your cutout with one of the Adobe free mobile apps, probably even Fresco or Photoshop Fix or Mix(is that still a thing, Kev?)

Then create a transparent PNG to put into express.

If you want to stay "express only", you could (really difficult) go into the EDIT mode and paint out the remaining greenscreen pixels that remain in your image.