What does a sulis' icewalk alternative trait replace? Is it a continuous effect?
I believe it changes Elemental Assault, and Elemental resistances. The Suli can only use that element type for damage (cold in the case of icewalk), loses the other three types of resistance, and the icewalk ability is only active while Elemental Assault is active. Not a great deal.
It would only be active while the Suli's Elemental Assault is active.
A Suli's Energy Strike is sort of a replacement for their elemental assault and their elemental resistance. Basically, you choose one element to focus on, and lose the resistance 5 you have to all elements other than that. In addition, you also gain the appropriate elemental ability, active as long as the Suli's elemental assault ability is active.
So the basic Suli would have Resistance 5 to fire, cold, acid, and electricity. They could also use elemental assault to add 1d6 of any energy type they want to their attacks.
To have Icewalk, a Suli gives up their resistance to fire, acid, and electricity, and when they use their elemental assault, they can only add cold damage to it. However, while elemental assault is active, they also gain the icewalk ability.
Hope that helps.
Edit: Ha, ninja'd by a minute, after 3 and a half hours. Weird.