Can someone that just started using a CPAP get hallucinations from it?

Not usually. She may be incorporating new experiences using CPAP - noise, air pressure, mask/nasal pads on face... - into her dreams. Give her some time to get used to it.

graygrammie: I took 3 mg when I tried it, but that was apparently too much for me. If I ever decide to try it again, it will be 1 mg.

KPAR01: Perhaps the following may help you - https://www.cpap.com/blog/sleep-apnea-dreams-and-nightmares/

Disclaimer - The above info is not my authoring.

Also, I see that your mother unfortunately suffers from Alzheimer's, which may have a factor on your question.

Just an FYI: I don't use a CPAP but tried melatonin for sleep. I had horrific, vivid, violent nightmares. Scary! I'll likely never try it again, but if I do, it will be at the lowest possible dose.

She might be getting a better quality of sleep and actually dreaming. Since she is on lots of meds, is she also getting melatonin? Dreams may be more vivid.
Keep encouraging the cpap but do speak to her doctor about this new problem.

She could be remembering dreams now that the CPAP is enabling her to enter and remain in REM.

I've been using a CPAP for several years now. No, I can't think of any reason why using one would cause hallucinations. It's giving you air, not depriving you of air.

Is she using it correctly? Were the settings custom set for her in an appointment with her actual unit and a certified sleep technician? Is it working properly?

I never heard of such a thing myself. But, there is a thread on Cpaptalk.com whereby the poster says he was diagnosed with psychosis and medicated for it, which stopped the hallucinations. As soon as he started using a CPAP machine, they came back! I notice you say mom suffers from Alzheimer's/Dementia which likely translates to a form of psychosis. Is she taking anti-psychotic meds?

Here is the link to the discussion I mentioned:

https://www.cpaptalk.com/viewtopic/t186992/CPAP-Making-Me-Hallucinate-During-Daytime.html

Here is another link to read, Obstructive Sleep Apnea, Psychotic Symptoms Common in Alzheimer’s Disease Patients:

https://psychnews.psychiatryonline.org/doi/full/10.1176/appi.pn.2023.05.5.5

Speak to moms Alzheimer's doctor about this......the CPAP mask may have to go.

Good luck to you.

I looked it up and I saw a couple of instances where people did have hallucinations but they had mental problems too. Call her doctor.