Can I be written up off if I was off of the clock and policy was not followed?

You have no legal right to have this write-up thrown out of your employee file. That is unless you have protection against such an action under the terms of an employment contract or union/collective bargaining agreement. The fact is that most employment is "at will". This means that, absent some form of actionable discrimination, a company can set the conditions of the workplace much as it sees fit. This includes what is done with employee personnel files. Accordingly, unless you can get one of your manager's superiors to agree to remove this write-up, you have no other recourse in this situation.

No, you cannot have the write-up thrown out of your file unless a higher-level manager agrees do that, overruling the manager in question. Legally, a manager can write you up or otherwise discipline you (up to and including firing you) for any reason and for things you do off clock as well as on (e.g. if you are rude to a manager on your free time, he or she can fire you). This is an aspect of "employment at will," which is the law of the land: employees have essentially no rights at work, because it is all at the "will" of the employer. The only limitation on a manager's power to discripline you are those imposed by more senior managers or owners of the company.