voltage divider circuit for dc motor
Yes, the effective electrical resistance of the motor changes very much with a varying electrical load. Plus, when your motor is drawing 6V &times; 150mA = 0.9W, your resistor is going to be dissipating 3V &times; 150mA = 0.45W, wasting 1/3 of your available energy. But when the motor is lightly loaded, it will be getting more voltage than it wants.

A small switching regulator to convert your 9V source to 6V for the motor would be a better choice, and more efficient as well.