Watering too much or too litle is the biggest killer of indor plants. If the stuff on your container soil is soft, it is mold, and that means you are watering too frequently. Wait until the soil is dry to one inch down before watering again. If the stuff is crusty, you could be watering too little. Or it could be a build-up of salts from the water itself, especially if it is white. In that case, ease the plant gently out of the pot, set it aside, and scrub the pot with nine parts water to one part bleach. Then rinse thoroughly with clear water. This is a good time to repot if the roots need more room.