What happens to the opponent's Energy if I use Krookodile's "Piston Headbutt" attack, but my opponent's Pokemon has Silver Mirror attached to it?
If Silver Mirror is attached to the Defending Pokemon and it is not a Pokemon-EX you cannot move an Energy off of it. If the Defending Pokemon does not have Silver Mirror attached, a Benched non-EX Pokemon with Silver Mirror is a valid target for the choice of where to move the Energy to, but since Silver Mirror prevents all effects of attacks done by Team Plasma Pokemon the Energy would be discarded rather than attached to the Benched Pokemon.
Piston HeadbuttSilver Mirror