In the manual it says 12V external power is required.
Can it be powered by PCIe without 12V external power?
As from this documentation:
It is stated that the Mustang-V100-MX8 requires 12v DC power.
You should use power cable with 6 pin connectors from the system if applicable.
Or add 4pin to 6 pin PCIe power adapter to connect to the power connector of Mustang-V100-MX8
In short, you need the 12V power, otherwise the hardware would go under voltage and could not perform efficiently.