The pot is more than big enough for that size Cym. That Cym. can stay in that pot for another 2 years at least.
If the pot gets too small, you'll get the roots coming out of the drainage holes and wrap tightly around inside the pot. Not flying roots. That usually happens due to watering issues.
The roots are coming up because of the water. They are going to where the moisture is. If you had moss on top, that explain why you are getting roots on top. Cyms. don't need moss in or on top of the pot. That would also explain the rot problem since the moss will stay wet too long around the new growths.
That's my opinion but someone else may have more to add. Hope that helps.
