Can a hospital case worker take my power away from making decisions for my elderly mom in CA?

The answer is no. Like said she can report you to APS but Social Workers don't have that power. But, you do need MPOA to make decisions for Mom.

Esther, are you still here?

Generally it has to be the principal who changes the POA, if the principal is mentally competent to make decisions.

What kind of "power" do you have? Medical Proxy? Financial Power of Attorney? Did your mother sign these documents in the present of a notary? Are they on file at the hospital?

Is there a specific decision that the case worker is making in spite of your objections, or are you being excluded from everything? What reason was given?

First, what powers of attorney has your mother executed? Living Will, or Medical POA? Durable POA?

The case worker her/himself can't "take" your power away, but she/he can bring in APS or institute guardian or conservatorship actions if necessary.

But there has to be justification. And the hospital doesn't necessarily want the authority; they just want to ensure that proper care is in place.