tell tales
• gossip about or reveal another person's secrets or wrongdoings.
• "she sparked a family row after telling tales about her mother on a TV show"
telltale
noun
• a person, especially a child, who reports others' wrongdoings or reveals their secrets.
Similar:
tattletale,
blabbermouth,
blabber,
loud mouth,
snitch,
squealer,
sneak,
clype,
pimp,
talebearer,
• a device or object that automatically gives a visual indication of the state or presence of something.
