This section of the code disqualifies mediators and arbitrators from testifying in court regarding mediations or arbitrations, except where a statement or conduct could "(a) give rise to civil or criminal contempt, (b) constitute a crime, (c) be the subject of investigation by the State Bar or Commission on Judicial Performance, or (d) give rise to disqualification proceedings." This section is also not applicable to mediators in child custody mediation cases.
California Evidence Code Section 703.5: Competency
Legislative Counsel of California. Jan. 1, 1970https://leginfo.legislature.ca.gov/faces/codes_displaySection.xhtml?sectionNum=703.5.&lawCode=EVID