User blog comment:Sky-Dragoon-Twilight/Weisslogia and Skiadrum, Alive or Dead?/@comment-18654001-20141208200051/@comment-9595671-20141211044241

It might make sense for Weisslogia on the ethereal thing, but not as much for Skiadrum. Since Dragon Slayers' abilities are based on the element of their dragon teacher and Sting has shown the ability to become a shadow moving on the ground that means Skiadrum can do it too.

Igneel did use a Fire Dragon Iron Fist like attack earlier after all. So while that does make sense, the nature of Rogue's power makes this theory have the one major hole of the fact Skiadrum most likely has an ability that makes it look like this when he actually isn't.