"I recently moved to MO from CA"
If you've literally moved to MO and given up your home in CA, then for 2020 you will file as a part-year resident in both CA and MO.
Once you become a resident of MO, the income you earn by working remotely in MO is not taxable by CA. It is fully taxable by your new home state of MO. The issue is where you actually (physically) work, not where your employer is located. CA can only tax non-residents on work income earned from work actually performed in CA. The only exception would be if you were paid as an independent contractor and you received a 1099-MISC at year's end instead of a W-2 .
You become a resident of MO for tax purposes on the day you begin living in your new domicile (your main, primary home) in MO. You remain a resident of CA - for tax purposes - until that day.
Finally, if your employer cannot or does not withhold MO taxes from your pay after your move, you should start making quarterly estimated tax payments to MO. Here's a link to the MO estimated tax form and instructions:
https://dor.mo.gov/forms/MO-1040ES_2020.pdf
MO estimated taxes can also be paid online if you prefer:
https://dor.mo.gov/personal/payonline.php
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
