Answer: If you've had the plant for a while, the white "mold" on the soil surface may be a buildup of minerals from the water and/or fertilizers. Scrape what you can off the soil surface, then flush the soil by saturating it and letting it drain; do this several times. Be sure to let it drain well at the end. Repeat the process every few months. If the plant is relatively new, or newly repotted, the white substance may indeed be mold, which indicates overly moist conditions. Allow the soil to dry out somewhat between waterings, and place the plant where it will receive good air circulation.
Yellowing foliage can be caused by a variety of things: over- or underwatering; exposure to a warm draft such as from a heating vent; lack of humidity; physical damage to the supporting stem by bruising, pruning or a training tie becoming too tight; a need for repotting into a larger pot; or by pests.
Look carefully for webbing on the undersides of the leaves which would indicate a spider mite problem. If you find these, wash the plant in a spray of lukewarm water twice a week and spray with insecticidal soap according to the label instructions. Remove any browning leaves and webbing as soon as you see them. Increase humidity around the plant if possible; a pebble tray filled with water works well for this.
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