User blog comment:Hyperborean/"The House of Hades" – official expectations blog/@comment-3146930-20130601104422/@comment-3146930-20130601131706

I think the bluish glow you are seeing is the background behind Percy (well, in front of Percy as we are looking from behind him). The blue stuff at the bottom could be some kind of water, but to me it seems like whatever-it-is is trying to grab Percy and Annabeth, as if Percy was just trying to control it, it wouldn't be trying to climb up both of them. It just doesn't really look like water in my opinion. It seems more like mist or fog. Maybe that is what the essence of monsters looks like and they are after Percy and Annabeth.