What’s the concern? The drooping? Or is it developing mold or a fungus? Can’t quite tell from the photo.
More details, Please! :)
Fairy Castle Cactus Features: An Overview
This cactus belongs to the Acanthocereus genus that contains exactly 14 species of cacti. Fairy castle cactus is the somewhat representative cultivar of this genus.
Fairy castle cactus is a tall, columnar perennial plant. This cactus can reach up to 6 feet (2 m) in height. It tends to grow at a very slow pace with time and produces numerous branches that provide an attractive silhouette.
The plant consists of many tall, four-sided, light to dark green stems that feature wool-like spines along each plane. Its limbs turn woody and brownish with age.
In general, the Fairy castle cactus exhibits blossoms quite rarely. But when it blooms, the flowers appear only during the night and remain open until dawn. The flowers do not usually show up until the cactus is ten years old or even more.
The showy flowers of Fairy castle cactus have lots of delicate, long, pointy, and white petals. They have dense, creamy white or yellowish centres (pistils).
Once its blooming period has ended, the Fairy castle cactus may bear fruits where the blooms were before. The fruits are tiny, shiny, spiny, ovoid to oblong, reddish, and measure about 2 inches (5 cm) in length.
https://gardenbeast.com/fairy-castle-cactus-guide/