Hi Eden,
You ask a very good question. I will just start with a disclaimer that I am not a believer in spirits, but I also have no proof they don't exist - so I choose to believe it is a possibility. I have experienced some unexplainable things myself (ie, my mother and I both shared a simultaneous hallucination of my deceased father walking by her pool in her gated backyard).
Mental health science and science, in general, don't study the metaphysical. They focus on the obeservable and repeatable world. Often you will see these disciplines speak out against the metaphysical world, but that is an overreach of their knowledge and authority. It is not their area of focus or knowledge.
Here are some of the major criteria of all mental illnesses that are important to help you distinguish what would be considered problematic or not:
1. The experience is distressing for you.
2. The experience is interrupting your ability to complete your daily obligations (ie., go to work, eat, sleep)
3. You continue to engage or spontaneously experience even when you are aware the experience is causing you harm or hold you back.
These three criteria are on every single mental health diagnosis in the DSM V TR (the latest edition) and the conditions must be met to warrant a diagnosis. So if none of these apply to you, then you do not need to be concerned that you have a mental illness. Spirituality is found amonst every society, so long as you are not harming yourself or others - it's no one's place to judge.