Yersl:
The reason people do not call chalav stam “chalav akum” is because R’ Moshe holds that any milk in America is considered cholov yisroel (since there are laws that prohibit anything besides cow’s milk being in a container marked “milk”). Therefore, it is called cholov stam only to differentiate between cholov yisroel according to everybody and cholov yisroel only according to R’ Moshe.
(I only read the OP of this thread, so I apologize if this comment is repeating things already said.)