From the Super Mario Wiki
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A Ghost Heart is a heart in the game Luigi's Mansion. When the player flashes the light on a ghost, it will be stunned and will show its heart for some time. Then the player can suck the ghost up in the Poltergust 3000; however, it can only be done when its heart is showing.
Portrait Ghosts are harder to catch as they won't show their hearts so easily. The player needs to find their weakness and then the player can shine the light on the ghost and suck them into the vacuum.