the pot is porous and will act as a filter for any soluble elements present in the soil.

Calcium carbonate will be there in trace, as will nitrates of potassium and some phosphates of same.
it`s totaly harmless (although unsightly if you`re fussy), you could scrape them off and re add them to your water again, or collect them and use them as a plant feed "Waste not, Want not"

it`s just leeched out nutrients, that crystalise as the pot has a large surface area to allow evaporation of the water used to carry these chems.