Please tell me what can cause hallucinations in the elderly?

Many possibilities : The patient needs a careful examination ; possibly additional tests, as there are many possibilities. These include medication side-effects, systemic infection, stroke, seizure, head injury, brain tumor, sensory deprivation (from poor eyesight and hearing), some dementias, and others.

Many: Underlying dementia predisposes, as does visual or hearing deficits. Certain types of dementia more commonly cause hallucinations- particularly lewy body dementia- but all patients with dementia may have hallucinations at some point.