If it were mine I would use something like neem or all seasons oil and gently hit it with a toothbrush weekly for a few weeks. That is a LOT of build up! Consider never watering any of it except the substrate surface with a small watering can. Filtered water won't stop that it will just reduce it. We have a very good whole house filtration system and I frequently just use watering cans and never do a foliar spray. My Wife has a few decent pots inside and outside for the Summer - those get Distilled water as it is inexpensive and not much is needed for the small stuff.
Gotdammm that is awesome, super jealous over here.
For hard water I really only "clean it" without the tree in it and then with vinegar and a stiff plastic brush. When there is a tree in the pot, olive oil rubs