This is totally normal with these grafted cactus. The red Gymnocalycium cactus was grafted to the green cactus, these moon cactus have a shelf life of no more than a few yrs. I’d even guess to say the norm is about two yrs. The bottom green cactus will live without the top colorful cactus, but not visa versa because the bright yellow, red, or pink Gymnocalycium lacks chlorophyll. There are moon cactus with dark purple Gymnocalycium cactus that are able to survive without the bottom green guy.

The cactus that survived is a Hylocereus and yours looks fine, this type of cactus are known to grow tons of aerial roots. They’re epiphytic “jungle“ type cactus. Here’s some info about Hylocereus.