Who higher than POA?

You could file for guardianship. See an elder law attorney, may be possible on an emergency basis.

But if mom is competent she has the choice.

The grantor, if still mentally competent, always has the final say.
Other than that to overrule a POA you would need to go to court to fight for guardianship (and going by your other post it sounds like it WOULD be a fight)

Are you asking who has more authority than the PoA? Who can overrule it? I don't think anyone, unless they are abusing the PoA. The govt. (social services/the county), the courts if necessary. But it's through a legal process.