If I have been a teaching artist in schools can I be grandfathered in and receive a credential?

No.  Credentialed teachers in California must have gone through a credential program.  Having a BA, BFA, MA, MFA, PhD, or DFA does not count.  Teaching artists are often excellent teachers, but Education Code requires that teachers also have training in Special Education, Emergent Bilinguals, Education Code, Educational Foundations, etc.  The only possible exception is if you have been teaching dance or theatre in a private school for six or more years.  More information is at https://www.ctc.ca.gov/