What is the name of your state (only U.S. law)? Washington.
I'm 18 and want to move in with a friend, but I have been told that my parents will call the police on me if I try to leave. Can they do that? Can they really stop me?
No, they really cannot stop you from leaving, at 18 you are legally an adult. However, they can make life complicated for you by not letting you take stuff with you, not letting you have legal documents (birth certificate, social security card etc) that might be in their possession and not cooperating with FASFA applications for you to get financial aid for college.
They can CALL the police against you because anybody can call the police for anything, but you cannot be arrested or detained for leaving home since you are a legal adult. Just make sure that you have fully considered everything before taking the step of moving out in a hostile manner.
If things are so bad that you are willing to walk out with nothing but the clothes on your back and whatever is in your pockets then that is one thing...if you are not willing to do that, then its another.