The imp has the Shapechanger trait:
The imp can use its action to polymorph into a beast form that resembles a rat (speed 20 ft.), a raven (20 ft., fly 60 ft.), or a spider (20 ft., climb 20 ft.), or back into its true form. Its statistics are the same in each form, except for the speed changes noted. Any equipment it is wearing or carrying isn't transformed. It reverts to its true form if it dies.
As far as I can see, it only mentions that the statistics remain the same (aside from the speed).
Can an imp still use its Invisibility action while shapechanged, or remain invisible while shapechanged?