Is Glaceon's "Freeze Zone" Ability stackable?
Yes, it is. Your Team Plasma Pokemon's Retreat Cost is two less energy per each Pokemon with the Freeze Zone Ability you have in play.
Source: Plasma Freeze FAQ (2013-05-16), TPCi Rules Team (2013-05-16)

Relevant cards

Note: All applicable cards may not be listed below.
BW9_23