puppy mouthing strangers. What do i do?

Hi, my puppy is almost 8 months and he's over teething, but everytime someone comes over to pet him, he gets excited and opens his mouth. I didn't see any aggression but I'm concerned about it. The nipping has stopped with him but he seems to think that its okay to mouth. I never taught him how to stop bitting because I didn't know how. I yelped and redirected but yelping seems to make him more excited. What do I do when strangers are coming to pet him? Is this normal?