Shaykh Tantanawi (rahmatullahi alaih) was asked:

Is it permitted to say such and such (alaihi salam), if he wasn’t messenger?

Answer: First of all let it be salat wa sallam upon others than prophet by obeying to messenger, alaihi salat wa sallam. We say Sallalahu ala Muhammad wa ala Alihi wa Ashabihi. As for signification for salam of someone  especially, then nearly majority of scholars on the opinion that it is not permitted. (ilal akhir)

Source: “Fatawa Ali Tantanawi” p 283, Darul Manarah