Charles Bonnet Syndrome

Charles Bonnet syndrome causes a person whose vision has started to deteriorate to see things that aren't real (hallucinations).

Up to half of all people with macular disease experience visual hallucinations, known as Charles Bonnet Syndrome. Many worry unnecessarily that there is something wrong with their mind. But it is important to understand that these hallucinations are a natural experience, not a sign of mental illness.  

  https://www.nhs.uk/conditions/charles-bonnet-syndrome/