Keep a log of any contact in case you need to go to the police and if he follows you again when out in public, go the nearest security guard and let them know you’re being harassed. It may be an over the top reaction after the first time but it sounds like the start of a persistent stalker

I mean, you'd have to live under a rock to not know the wife of Fred Flintstone.

He doesn’t need to know your name — he just refers to you as “my girlfriend.”

Hmm, sounds like the type who’d use the classic, “Are you an angel? ‘Cause I’d like to take you back to my place and show you my celebrity toenail clippings!”

You didn't have to. He clearly saw it while you were on Instagram.

Probably took a glance at your phone while you were too busy browsing IG. Simply deny the request and it shall be fine.

What is worrying is 63 stalkers seem to think she deserved being followed by a creeper.

Geotag

Maybe he made a picture of your face and used Google reverse image search

You probably came up on his Instagram and he recognized you. I wouldn't worry about it.