If a person is said to be brain dead, can they moan or lightly squeeze finger?

Usually not: The exact definition of brain death (in fact brain stem death) is varied but usually comprises the following no spontaneous movement , no respiration (i.e. Totally dependent to a respirator) no response to pain, fixed dilated pupils. Certain primitive reflexes may persist.

No: Dead means dead. Brain death doesn't mean coma (asleep-like unresponsiveness) where reflexes and very primitive responses are still possible. A brain-dead person can no more moan or lightly squeeze a finger than a completely dead person. They can be disconnected from life support without anyone's comsent because brain-dead is legally dead in most or all of the US.