In the intro to the movie Pacific Rim, they mention Kaiju Blue being somewhat of a problem after a kaiju was taken down. I didn't quite understand what it was or why it was a problem. What was it?
It got elaborated in wikia pretty well:
"Kaiju Blue" is the Blood of the monsterous Kaiju creatures. It is also highly toxic to the Earth and Humans. It can instantly poison the land it falls on. Everything gets stained, Peoples skin, their nails, everything. and if it gets into the human system, the suferer will begin to cough up blue vapour. -
In addition to AnkitSharma's correct answer to the question of what it actually was I'd just like to add another viewpoint that occurred to me in the wake of the recent Godzilla-hype.
Radiation has always been a strong element of the Godzilla-franchise, especially Godzilla's nature as a radioactive monster and his meaning as a symbol for the dangers of nuclear weapons and radioactivity in general. So since the obvious influence of Godzilla on Pacific Rim is undenyable, being the Kaiju-movie(s) par excellence, it may very well be that Kaiju Blue and its devastating and toxic effects are also a reference/hommage to exactly this strong radiation aspect of the Godzilla-movies and other Kaiju-movies they spawned.