Conrad Hilton's ex, Hunter Daily Salomon, is living in fear ... because she believes he has serious mental health issues.
The 21-year-old actress was in L.A. Sunday -- the day after Hilton was arrested for trying to break into her mother, E.G. Daily's house, and stealing her father's Bentley.
You can tell ... Hunter doesn't want to trash Conrad, but she's also clearly terrified. As E.G. told us ... she and her daughter think it's time for Conrad to get professional help.