The white or tan crust on the surface of house plant potting mix is a build up of soluble salts. Excess soluble salts burn foliage, damage roots and lead to problems with water uptake. Wilting, yellowing, and marginal and tip burn of leaves, also called scorching, are symptoms of excess soluble salts.
Soluble salts also accumulate on the outside of clay pots, around drainage holes on pots and even on the stems of plants. Salts come from the potting mix, fertilizers or high salts in the water used to irrigate the plants. These salts rise to the surface of the potting mix by a process called capillary action, especially when the pot sits in a saucer containing excess drainage water. Excess salts cannot drain through the pot when saucers keep water under the container because the water is re-absorbed. Leaching salts from the soil is an easy remedy for plants affected by excess salt.
For more information, please see the PlantTalk script #1338, Whitish crust on potting mixes.