Can I sue a co-worker for libel/slander without suing the employer in state of CA

The answer is: You can sue the person who wrote or said untrue defamatory comments about you to other people. So if that person is a co-worker or your employer, you would sue the one who uttered or wrote the defamatory comments. The first thing you should do, however, is consult with a lawyer. Defamation litigation is very expensive and complicated. For example, the defendant might have the right to file something called a SLAPP motion which will delay your lawsuit, and might end with you being liable for their attorney's fees if the motion is handled incorrectly. Also, depending on the context of the defamatory statements, the defendant might be able to assert a privillege in their defense. That is why your first step should be to consult with an attorney as soon as possible. Nothing herein constitutes the formation of an attorney/client relationship.