That looks like it might be a fungus. A fungicide will stop it from spreading but the spots will remain. Hopefully you are only watering when totally dried out, they are so much easier to grow outside if you have the climate for it.

Probably in response to the soil, which should be more porous. Pot culture is different for plants than planted in the ground, and you need to do things a little bit more carefully in pots, in that plants in the ground are a lot less fussy than those in pots.