You Do Need power plug adapter while traveling from Saint Lucia to Guam.

Here is Why You Need An Adapter

The outlet and plug type used in Guam are A and B and in Saint Lucia is G. This is how they look like.

Saint Lucia

In Saint Lucia, outlets of type G are in use.

Type G

Guam

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

Type A

Type B