You'll really need to give us some good images. Your description could be one or more of many different things.

The Ponytail Palm (" Beaucarnea recurvata") is not a true palm but a member of the Agave (think Aloes) family and as such grows in a dryer than normal soil. It sounds like you are watering the plant too much and it is developing a mold common in those plants that normally grow in very arid soils will get if the soil they are now growing in is watered too often.