This could be due to a few different reasons. The plant looks to be fairly large in a smaller container. When this is the case, it becomes more difficult to water the plant appropriately. I can see what looks like a bowl or saucer underneath the pot. This may help with hydrating the older potting media that the plant is in, but it may also hold excess water that the plant does not need. Here are some helpful tips for watering Christmas cacti appropriately:

• Place plants in a sink to drain when watering.
• Remove excess water from saucers and decorative pots. Excess water may result in dropping flower buds, wilting and root rot.
• Allow soil to dry out between watering.
• Plants won't grow well in heavy, wet soils.
  • Do not allow plants to sit in excess water after rainfall or watering.
  • If water has been abundant (e.g heavy rainfall) and plants appear wilted, they may be suffering from too much water.
  • Allow the plants to dry out, check the container for adequate drainage and relocate the plant to a drier, part shade site.
• After the plants have finished blooming, water less frequently, increasing again in spring or early summer when the plants resume more active growth.

A couple of other reasons you might see yellowing and pale growth could be due to sun exposure or inadequate fertilizer. Christmas cacti prefer about 3-6 hours of sun per day and excessive sunlight may result in pale green branches, drought and sunburn. Additionally, inadequate fertilizing in fall leading up to flowering could cause some pale, chlorotic growth. It's also recommended to fertilize in June- August with a houseplant fertilizer at half strength. 

UMN Extension and Univ. of Wisconsin Extension have great guides and MSU Extension has a helpful article on Christmas cacti care as well:

https://extension.umn.edu/houseplants/holiday-cacti

https://hort.extension.wisc.edu/articles/holiday-cactus/

https://www.canr.msu.edu/news/how_to_care_for_and_reflower_your_christmas_cactus