You Do Need power plug adapter while traveling from Guam to Sri Lanka.

Here is Why You Need An Adapter

The outlet and plug type used in Sri Lanka is G 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

Sri Lanka

In Sri Lanka, outlets of type G are in use.

Type G