Based upon the facts set out in your question, I believe that you are employed at will. Most people are. Your employer may reduce your hours and your rate of pay at its discretion.
In CA, employment is presumed to be at-will, which means that you or your employer may terminate the relationship at any time, with or without cause. As such, your employer may also change the terms of the employment prospectively. Although your employer may reduce your hourly rate, it cannot go below $8/hour which is the current CA minimum wage.