Although I don't disagree with Mike regarding the dangers of leaking batteries, I don't believe that's what you're seeing. My best guess is that it's some kind of OCA, or Optically Clear Adhesive used to bond the LCD to the digitizer, and for some reason, possibly age and/or heat, it has liquified and is leaking out of the crack. The same thing happened to two Acer tablets that I have; after being in storage for somewhere between 6 months and a year, when I got them back out they had both come up with leaking displays. Here's the question I posted about the subject.
[post|827519|My LCD is Leaking? - Acer Icona W510 - iFixit|new_window=true]
As to what it is, or if it's dangerous, I still have no idea. I suffered no ill effects from cleaning it up, but I did try not to get it on me either. It doesn't have any detectable odor that I could discern.