I am not a health professional but I talked with a friend who is a clinical neurologist about this.

She commented that pineal gland cysts are quite common (about 2% of the population have them), 6 mm is a normal size and in the overwhelming majority of cases they are benign.

She suggested that you get your doctors to do more detailed imaging - in the very remote chance that it is malignant, the sooner treatment starts the better.