Am I eligible for CA renters tax credit if I lived in CA for 178 days in 2018.

No, you must be a full-year resident. 

You must meet all of the following to qualify:

1. You were a California resident for the entire year.
2. Your California adjusted gross income (AGI) is:
   • $41,641 or less if your filing status is single or married/RDP filing separately.
   • $83,282 or less if you are married/RDP filing jointly, head of household, or qualified widow(er).
3. You paid rent for at least half the year for property in California that was your principal residence.
4. The property you rented was not exempt from California property tax.
5. You did not live with another person for more than half the year (such as a parent) who claimed you as a dependent.
6. You were not a minor living with and under the care of a parent, foster parent, or legal guardian.
7. You or your spouse/RDP were not granted a homeowner's property tax exemption during the tax year.

• You may still qualify for the credit if your spouse/RDP claimed a homeowner's exemption and you maintained a separate residence for the entire year.'

https://www.ftb.ca.gov/individuals/faq/ivr/203.shtml