Suffolklad
I see what you mean about that green cap looking like it holds it on, however I dont think this is the case.
First thing is to get the caliper and carrier off as one piece, the bolts for the carrier are often very difficult to loosen. The trick is with stuck carrier bolts is not to hit your spanner with a hammer, but get the longest handle one you have, or extend the handle with pipe, and put strong continued pressure so you don't shear the top off.
Then hit the disk on the edge al the way around a few times, see if this helps. dont hit front and back too hard as you dont want to harm your hub or bearing. There may be a spot behind the disk you could use to lever with a bar. With any luck it will come.
I believe the green capwill more likely be protecting the grease in the hub, will probably pop off. My car runs with this exposed, have never had a problem with it.
Good luck!!...