You need the Silph Scope, which you get after defeating Giovanni in the Rocket Game Corner basement.
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you need to find the silph scope this will reveal the ghost to be a marowak you can then defeat it
You need the Silph Scope to find the ghost's identity bet you have to get it from Team Rocket's boss. When you defeat him, he will leave it behind. Get it and go to Pokemon Tower. Then go to the stairs where a ghost won't let you pass. You find out the ghost's identity, which is Marowak. Then defeat it and it will calm down and depart to the afterlife.
You need the Silph Scope to see it, then defeat it.
You will need to have the item called the silph scope which reveals the identity of the ghosts that ghost happens to be the deceased mother of a cubone aka Marowak you will have to defeat her in order to reach the top of Pokemon tower.
Haunter.... There isn't that many ghost types in firered besides the Gastly line which you can find in Lavender Town's Pokemon Tower