Calla leaves and flowers grow out of rhizomes without any stems. Slime, or rot, often occurs at the soil line where the roots transition to leaf shoots. Rot sets in when the potted calla has soggy soil. Water your potted calla only when the top 1 inch of soil feels dry to the touch. Damp soil encourages healthy growth, unlike soggy conditions that contribute to rot and die back. Cut out any slimy leaf shoots and reduce your watering frequency to rejuvenate the plant. However, your potted calla usually will not recover if it has slimy and decaying roots.
Use a well-drained potting mix as your medium, rather than garden soil. Although full of nutrients and organic matter, garden soil does not drain properly in a container. Poor soil drainage reduces available oxygen to calla roots, causing suffocation. Rot sets in as soggy conditions persist as well. Select a container with drainage holes to avoid trapped water inside. Exposure to full sun or partial shade near a window allows soil moisture to evaporate normally from the container, while allowing roots to absorb the water effectively.
Callas bloom in the summer and die back slowly in the fall. Slimy conditions are not normal for a potted calla at this time. In fact, foliage withering should display only yellow and papery leaves six weeks after the plant's last flower growth. Avoid any slimy issues by cutting the spent flowers from the plant. Mold and bacteria grow on decaying flowers and negatively affect the rhizome below. Allow the foliage to remain for photosynthesis processes to encourage lush flowers next growing season.
Your slimy calla could have originated from the rhizome itself. Fungi spores hidden in the rhizome can reproduce quickly in wet soil conditions. To avoid future slimy calla issues, purchase rhizomes that are stiff without any soft spots. Fungal infections give the rhizome a spongy feel with some slime already forming on the surface. Although fungicides are available, it is best to avoid diseased rhizomes rather than rejuvenating them with chemicals. The calla will not be as healthy as a disease-free specimen.