Behind the stalker account..

Ladies (and fellas) - I need your help regarding a situation I’m in. This guy I’m dating (for a year now) has someone from his past messaging me from an anonymous account every so often. This person’s message is very cryptic and unsettling; they’ve even reached out to my best friend revealing some false information. But also know details about me... I am creeped out. I’d appreciate if you could help me out on how to find who they may be. What type of app I should download, etc... they continuously come and go messaging me out of nowhere. I feel uncomfortable and not safe.