This is actually a gray area right now. It is not taxed by California but may be taxable by the IRS. See
https://ktla.com/news/california/do-you-need-to-report-your-inflation-relief-check-when-filing-taxes...
for what appears to be the current thinking on this.
For now it is best to consider it taxable and, if later deemed nontaxable by the IRS (or the courts), file an amended return for a refund or, possibly, sit back and wait for the IRS to do it on its own if they say they would do it automatically.
