off his guard vs off-guard

off his guard and off-guard both are adjectives.

Word	Noun	Adjective	Verb	Adverb
off his guard	No	Yes	No	No
off-guard	No	Yes	No	No

As adjectives, off his guard and off-guard are synonyms defined as:

Other synonyms of off his guard include napping, off guard, off her guard, off one's guard, off your guard.

off his guard (adjective)	off-guard (adjective)
not prepared or vigilant	not prepared or vigilant