Nametag
From Starbounder - Starbound Wiki
Nametags are a display of a players name above their character. In multiplayer characters nametags appear for the first 5 seconds they're on the players screen, then fade away. At any time the status button (Alt Key) can be held to see nametags. They will remain visible as long as the button is held.
It's been confirmed that player nametags won't appear in the dark, adding an added element of stealth to PvP combat.[1]