Walmart has an Open Door Communications process which is the most direct way to voice any concern to a manager.
However if you believe that your immediate manager is involved in the problem, you can discuss the issue with Walmart Global Ethics Helpline; just dial 1-800-WM-ETHIC (1-800-963-8442) or 1-800-805-9121 for Canada (French). Or you can send them an email at firstname.lastname@example.org.
The line is available 24 hours a day, seven days a week, and is equipped to handle most local languages. They are staffed by an organization not affiliated with Walmart, and to the extent possible (and in conformity with local regulations) callers may not have to give their name.