When the kid called Laurence Fishburne a "quack" and Fishburne looked slightly puzzled by the word, I guessed that Fishburne was an alien because he didn't know what a quack was. I rarely figure out movies that early on.
The character's flustered reaction was easily explainable as a simple disbelief that anyone would consider him a "quack". Your jumping to such an outrageous conclusion, despite the fact that it actually ended up being true, is not evidence that the film was so transparent.
Admittedly, I had several theories percolating throughout the course of the movie (with yours being counted among them) but to have cemented any of my suspicions on such an ambiguous scene would have shown fairly severe myopia. Fishburne's reaction is definitely a bit odd, no doubt about it, yet it hardly shouted "ALIEN!".
We all bring our own biases to situations we witness so here's my (possibly) outrageous conclusion to yours: You jumped the gun... even if you were ultimately right.
What's wrong with jumping the gun? Especially if he hit the target. I started getting the whole alien vibe when watching the guys in HazMat suits washing down walls and hosing floors with what looked like Betadine. Couldn't really say why. Maybe that same feeling that keeps a woman from getting into a car with a guy because she thinks he has rapey eyes.
Fishoburne acts in a non human way the entire time, I thought. His very calm way of talking, and deliberate way of moving, flat affect, no emotion. I thought he was a robot early on.
I thought *something* was amiss when that happened, but I hadn't figured it out at that point, and I am usually good at picking up 'tells' early on in a movie.
"Much communication in a motion, without conversation or a notion"