The white crust you see on the pots is an accumulation of salts. The salts may have come from fertilizers used to feed the plants, or they may be minerals such as calcium and magnesium that are naturally dissolved in your water source. Pull the plant out of the pot while you clean the clay. An easy way to do this is to hold the pot upside down and tap the rim on the edge of a bench or table. This usually loosens the root ball so the plant will slide out intact. Use a wire brush to scrub the pots in a dilute bleach solution (1 part bleach to 9 parts water). Then rinse thoroughly with plain water to get rid of the bleach. If the root ball of the plant held together, you can slide it right back into the pot. Or if it's time to repot, place the plant in the next size larger container.