It sounds like you are doing good work! I have had hallucinations, though not frequently, only in times of very great disturbance. (PTSD/DDNOS if it helps you to know that about me, and I do experience parts/alters)

I think hallucinations can come with all sorts of conditions, and not just psychiatric issues either- bad drug interactions, sleep deprivation, etc. Their occurrence can be harmless or more worrying. They can change a diagnosis or simply become part of a bigger picture of yourself. Please tell your therapist! They sound very capable. In addition to providing diagnostic advice they can also help you with techniques to keep yourself centered when they happen.

As for me, we don't do anything about them, we rather just note that yes, they're scary, and I must be under more stress than I think.