This is an Epiphytic jungle cactus which is very thirsty, they require humidity & frequent watering. I use a pebble tray under my jungle cacti to help increase humidity around them.

I'd turn it out of the pot to check roots before doing anything else. Pls share a pic of the roots. I'd consider cutting off that yellowing part, depending on what you find below.

ETA: KarenS, NYC

thank you so much. I'm repoting it noe in new mix and I think I cut it back.

the roots were so dry idk if he'll make it but I'll give the best conditions feom now on. keep you updated if youd like.