off-guard vs off one's guard

off-guard and off one's guard both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off-guard	No	Yes	No	No
off one's guard	No	Yes	No	No

As adjectives, off-guard and off one's guard are synonyms defined as:

Other synonyms of off-guard include napping, off guard, off her guard, off his guard, off your guard.

off-guard (adjective)	off one's guard (adjective)
not prepared or vigilant	not prepared or vigilant