You Do Need power plug adapter while traveling from Isle of Man to Guam.

Here is Why You Need An Adapter

The outlet and plug type used in Guam are A and B and in Isle of Man are C and G. This is how they look like.

Isle of Man

In Isle of Man, outlets of type C and G are in use.

Type C

Type G

Guam

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

Type A

Type B