Small DC motor Heating up!
Without a datasheet it's really hard for all of us to help you. 



I will keep it simple:There are 2 possible reasons for overheating:


The motor is not intended for continuous duty operation.
The mechanical load is too high for the motor to handle which causes a high current to pass through the motor and heat it up.




possible soultion:

smaller fan means a smaller load. So you can try replacing the fan with a smaller one.

try to connect a small resistor is series. you should try and find the ideal value but you could try a value near 10ohm. the resistor will create a voltage drop and the fan will run at a lower voltage than 5V. The fan will run slower ofcourse



other solutions:


Heatsinking: think of a way to dissipate the heat. using heat sinks, air flow... etc
different power source than a USB port.