If I use Walrein's "Frigid Fangs" attack and on my opponent's turn they somehow move Walrein to the Bench, do they still need more than 2 Energy attached to their Pokemon to attack?
Yes. The effect resides on all of the opponent's Pokemon, not on Walrein, so moving Walrein to the Bench does not get rid of the effect.
Source: Surging Sparks FAQ; TPCi Rules Team (2024-11-07)