A wonderfully absurd hypothetical. 制服着込んだ外宇宙の使者 ('uniform-wearing emissary from outer space') is a compound modifier of magnificent length. 伴侶を求めて ('seeking a spouse') adds romantic purpose to the alien. 徘徊してる ('wandering about') is somewhat negative — implies aimless, suspicious movement. わけでもなかろうに uses the literary negative volitional of ある + に for 'it's not as if... (so why are you worrying?)'. The sentence is someone talking another person down from an absurd fear.