1. Why is my Christmas cactus wilted or limp?
The wilting or limp plant can be due to either too much sunlight or too little water. Give you plant bright indirect light and keep the soil moist with ample misting.
2. Why are the leaves turning brown?
If the leaves are turning brown and are mushy to touch, it is a classic sign of root rot due to overwatering. Let the soil dry out completely and water sparingly. If the plant does not look salvageable, take a segment cutting of healthy green parts and try to propagate.
3. Why is my plant covered in silvery-grey growth?
The silvery-grey matter covering your plant is a fungal infection called botrytis blight. Prune away the infected parts and keep it in a dry place with reduced humidity. Use a good insecticide for further cure and prevention.