Is it a house plant or outside? If it's inside, you should remove the white spongy thing, as it's probably mold. You may be over watering the pot. Take about an inch or two of soil off the top of the pot and replace it with fresh soil. Try sprinkling a little cinnamon powder on top of the new soil. It may inhibit the growth of new mold.
When you said the plant grew quickly, could it be it has outgrown its present pot?