We have absolutely no idea what that thing is. It could be in the Faron, Eldin, or Lanayru provinces. It could even be in the sky. But this is the first time I've seen a description of it. If someone could translate that it would definately shed some light on things.
EDIT: It probably is in the faron province since it is posted on the japanese sites faron section. Along with the siren realm and the flooded forest. Interesting....
I don't know about whatever video this was in, but the first I saw of it was on the Japanese SS website in the Faron Woods section:
http://www.nintendo.co.jp/wii/souj/world/FaronWoods/index.html
It was one of the pictures in the section that seem to discuss the Silent Realm. Click the Silent Realm image on the bottom right and it's one of the three screenshots in that section (if anyone could translate the text on that page, it may shed some light on this subject).
Given that it was part of the Silent Realm pictures, my first impression was that it was some unusual form of a Guardian, since we don't know how many forms of Guardians there are. On the other hand, something about it does look vaguely spirit-like, but again, without knowing what the text on the website is actually saying, we don't have any context to even begin speculating.
But if I recall corectly, the silent realm is blue when in "contact" with the tears, and naturally orange, I might be wrong, but I still believe that this entity is a Spirit, more precise Lanayrujust because she is blue, doesn't mean she's water. The Silent Realm(or whatever its called) seems to have a blue tent to it(http://www.zeldainformer.com/images/news/GI_920_1.jpg) maybe she is the creature that you give the light tears to(as the one above me has stated) but maybe she's the first entity you give them to, not Lanayru.