We looked at the photo and are not sure what you are dealing with (possible rot, fungal, etc. ) This plant does not like to be overwatered and likes good drainage. They are happy when crowded in their pot. You only need to transplant them when you see roots fill the pot (which could be every three years or so).

 We recommend that you remove the affected foliage and try not to overwater. In spring and summer, water plants when the soil surface becomes dry to the touch.
During their dormant period in the winter, Clivias may only need watering every three to four weeks. They should be kept in a well-draining pot. Do not water right at the middle stem. (Don't allow water to sit for a long period of time where the leaves meet the stem.) Water near the edge of the pot.
Here is more information on care, fertilization, how to get the plant to flower, etc.
 https://www.extension.iastate.edu/news/yard-and-garden-brighten-winter-clivia-plant

Marian