Why Is One Outlet Loose?
An outlet that moves when you plug something in shouldn't be ignored. The movement may involve the receptacle, the mounting, or the electrical box.
By Everyday Home Advice Editorial Team

An outlet that moves when you plug something in shouldn't be ignored. The movement may involve the receptacle, the mounting, or the electrical box.
Stop yanking plugs sideways
Movement can place additional stress on connections.
Is the faceplate loose or the outlet itself?
A loose cover plate is different from the receptacle moving inside the wall.
Look for other warning signs
Stop using the outlet and get help if you notice heat, sparks, a burning smell, discoloration, or crackling.
Don't work on energized wiring
Removing covers can expose electrical components. If repair involves the receptacle or wiring, the circuit must be handled safely.