cracked battery?
 

I have an electric club car with 6 batteries. on the 2 center batteries, there is a build up of a hard white substance. I believe it is battery acid, but it is not coming out the top. Could the 2 center batteries be cracked, and can they be replaced separately?

The batteries most likely are not cracked.
The best thing to do is remove all batteries making shure you draw a diagram of all the circuitry of the large and small wires.
Using a solution of baking soda and water clean the batteries and the framework of the battery compartment, be carful where you do this as the build up of this material is hazardes.
After all is clean and debris is removed, clean all terminals and reinstall according to your diagram, the should clear up your corrosion condition.