What is the output of the hall effect? Is it mA or mV?
There is no direct access to the Hall sensor with this device. The output signal is current (\$\pm 314\$ mA peak). The datasheet describes the device as a "Closed loop (compensated) current transducer" and based on details provided by the manufacturer, you can see that the cancellation current is the actual output (instead of simply amplifying the Hall sensor output):
Will need a resistor to have a potential difference and then feed to the A/D or micro?
If the ADC needs a voltage input, a current to voltage converter is needed. A resistor is the simplest one but more information is needed about the ADC to determine how the different current directions should be handled.
So will this decrease the accuracy?
Yes. Practically every single component, with its own tolerance, added to the signal path will add another source of uncertainty to calculate the overall precision of the circuit.