The ghosts in Pokémon Tower can't be seen until you have the Silph Scope. Even then, there is still the matter of fighting them.
Obtain the Silph Scope
The Silph Scope is found at the Rocket Hideout in Celadon City. It is hidden underneath the Rocket Game Center. Press the hidden switch behind a poster near the counter to reveal the hidden staircase to the Hideout, but you must defeat a Team Rocket Grunt first. Work your way through four floors of the Hideout and defeat Giovanni at the end, who gives you the Silph Scope.
Climb Pokémon Tower
With the Silph Scope in hand, return to Pokémon Tower. You can now identify and battle the ghost Pokémon that roam around there. The Tower ghosts are immune to Normal and Fight attack types, so come prepared. It is possible to catch a ghost and train it on the lower floors, as its Ghost and Dark attack types work well against the other ghosts.
Defeat Marowak on the sixth floor
Now that you have the Silph Scope, the ghost blocking the way on the sixth floor is revealed to be Marowak and can be fought. Marowak cannot be caught, but you must defeat it to access the rest of the Tower.