There are people who believe in the existence of ghosts.
Literal
Ghosts-of existence [object-を] believe-[ongoing-ている] people [also-も] exist-[animate-いる].
A common Japanese pattern: [attribute]-ている人もいる ('there are also people who are X-ing'). The も ('also, too') implies that there are also those who don't. Used for describing the existence of subgroups. 信じている here is the resulting-state ~ている — 'is in a state of believing.'