I would not do a "think twice before panicking" post. I might discuss it with my friend, then take a photo that's deliberately staged to be harmless buds being buds (good lighting, pleasant setting, smiling), and then post a "coming home from college, so excited to show my BFF Bob from my English Literature class around my neighborhood, he's visiting for two weeks and we love hikes and watching Grey's Anatomy!" thing, with the picture. This will hopefully prompt your neighbors to think "ah, that must be College Student Bob, Alice's friend" when they see him around, instead of "Scary Black Guy!!!" without specifically calling them out on the prejudices they might or might not have. (Let's be real: very Might, yup.)