Copyright is the cure to paranoia. Either with the Library of Congress which I do, or the Writer's Guild of America
www.wga.org. Even if you alter your screenplay no one can derive from it without your permission.
That being said. If you have a novel idea and a lousy screenplay, yes, someone could take that idea and write a really good screenplay around it. Unfortunately, ideas can not be copyrighted, like dinosaurs come back to life from genetic advances, Jurassic Park stole from Land of the Lost by Jules Verne dated, what 1800's?. Or the hero wins the girl after defeating the villian, any movie, any story, notably pre-Bible, the Odyssey.
As a screenplay writer, your job is to tell the story. A cool idea does not make a movie. Free exchange of your ideas in forums like this will make you into a better screenplay writer and open you to collaboration from everyone in the forum.
As an indie producer/director/director of photography/screenplay writer/editor/special effects editor/musician/ASCAP composer/musical engineer, when I encounter a novel idea, I give credit for the story idea. Fleshing out the story idea is the majority of the work, but you always give credit where it is due, why wouldn't you? The concept idea guy doesn't get much, I work for free doing my own movies so maybe I am not a good reference, but I still give credit and free food if you work on my movie.
Write and share, that is the way to greatness or at least to being at peace with your art.
A novel idea is great, but it is just an idea. I'm sure there are idea think tanks if all you do are ideas and not stories. Look into. However, if your bag is storytelling, work on the both the craft and the ideas. Leave the paranoia behind.
Most will give you credit for the idea, based on a story by {you}. However the real work is in the finished writing, then the herculean effort to produce the movie--"All movies are result of hard work and a few miracles." Claude LeLouche "A Man and A Woman" Academy Award Best Foreign Film, Academy Award Best Foreign Actress Anouk Aimee, Gran Prix Catholic Film Commission 1966.
Hope it helps, heals, Check.