Sadly your cactus kicked the bucket it rotted a common. Rookie mistake :) cactus do need a lot of water but not often to much water causes rot but u should get another water weekly and use gritty mix your cactus is a grafted one that were put together a mutant the top red one does not make chlorophyll so it was grafted so it would survive its is named moon cactus the bottom is the grafting stock and the top is the grafted cactus both are cacti grafting is man made and cacti don't look like that usually but u should get another Cactus here is my moon cactus Noah :)

Is there a way to save the red part?

OK I just read up on the moon cactus. What a funny thing that we do! (grafting these two guys together to make such a funny looking, yet endearing combo). Anyway, I know what the problem is which is, this poor little guy was placed outside right where it gets directly hit by the strong afternoon sun. That plus the cool temperatures of our wonderful SF summers at night probably contributed to its death. Sad! Maybe if I get ambitious I will try my hand at grafting the Gymnocalycium mihanovichii onto a new Hylocereus.

No only the the the top part gets sunburn it was rot