Mom has started seeing things that are not there. Should I just go along with it or not?

is your mom at home alone? Does she have dementia? Are you at all worried that, if she lives alone at home she will hallucinate and take off alone? Hallucinations and delusions aren't really something that goes away. I found with my mother that you need to enjoy the good days and pretty much wing it during the bad days. It helps if you have the support of your family and a good geriatrician or primary care doctor.

Sewingsh, have your Mom checked to see if she has an urinary tract infection, as those can cause seeing things that are not there.

That happened to my Dad, he started to see ants on the walls and in his food. The senior facility understood what he was going through, so they put down ant traps even though there weren't any ants. Then Dad was tested for a UTI, and sure enough he had one. Once the antibiotics started to work, the ants started to disappear.

Hallucinations are certainly common in dementia, but I agree with Zoe, have other possibilities checked out.

I would certainly consult your GP as there may be an organic cause for the hallucinations - i.e Infections can have a dramatic impact on the elderly and can cause Delirium, behavior changes etc. I would rule out any physical possibility first, then consider perhaps it is a progression.
Also make sure dehydrated isn't an issue.
All the best.