I think the answer is
Head & Shoulders
And the movie (which I haven't seen but found descriptions online)
I contain a deadly poison of powerful effect,
Head and Shoulders contains selenium in the form of selenium sulfide.
Don't worry you'll be safe from me, or so I should expect,
It is generally not present in large enough quantities to be harmful to humans.
My deadlyness is well reserved for beings otherworldly,
Head & Shoulders anti-dandruff shampoo is what is used to kill the aliens in the movie.
For you I could well be useful somewhat absurdly,
Selenium sulfide kills the scalp fungus Malassezia, which causes shedding of dry skin fragments.
The hero saw the answer sitting on a crowded table,
Ira figured out that selenium was poisonous to the aliens from its position on the periodic table.
He didn't know to find me but not so clever students were able,
Much to Ira's surprise, his dumbest students Deke and Danny recalled than selenium sulfide is the active ingredient in Head & Shoulders.
I'll save the world with pseudo-science and not as you'd expect,
The fact that selenium is poisonous to the aliens arises from analysis of the periodic table. They are nitrogen-based and the theory is that selenium will be poisonous to them as arsenic is to humans.
I didn't even pay the crew that won me such respect.
Apparently, the movie crew received no money from Head & Shoulders for product placement.