Has your Mum seen an Old Age Psychiatry specialist ?
If not, ask your GP for a referral.
When you say the Nursing Team, are they Mental Health Nurses ?
Again, if not, ask if your Mum can be referred to one.
If she is already being seen, make sure that the Doctor knows about this behaviour as well as the nurses.
It is not unusual for people with dementia to see people who are not there and medication can be very helpful.
I hope you can get some help with this as your situation is very hard to cope with.