You Do Need power plug adapter while traveling from Guam to South Africa.

Here is Why You Need An Adapter

The outlet and plug type used in South Africa are C, M and N and in Guam are A and B. This is how they look like.

Guam

In Guam, outlets of type A and B are in use.

Type A

Type B

South Africa

In South Africa, outlets of type C, M and N are in use.

Type C

Type M

Type N