My cactus is turning yellow and translucent (the picture makes it look brown, but it looks more yellow against sunlight). I've had it for over a year now and it's survived a dorm room and an apartment in the meantime. This cactus has been prone to tipping over (I don't think the roots were ever very strong) and the last time I had to upright it, I planted it deeper in the soil than before. I could be wrong, but I don't think it's root rot. The roots weren't mushy or smelly when I took the cactus out of the pot. I only water it every 2-3 weeks, the pot has drainage holes and it is planted in Bonsai Jack cactus soil. I have another smaller cactus and a bunch of succulents in the same soil with the same watering schedule and they seem to be doing fine. My best guess right now is too much sun exposure because of the yellow color, and/or underwatering because it is becoming see-through and less firm (not quite squishy, just thin and not plump). The windowsill in my room faces a different direction than last year, so that could change the amount of sun. Any insight would be appreciated!
That plant looks seriously ill, I doubt that it will survive. Rot does not always attack the roots first. The root system is very small for a plant of that age, which indicates that something has been wrong for a long time. Infected roots do not always turn mushy and smelly, they often just become dry, hollow and darker. It is spreading, right? If so it is certainly a disease. This type of grafted cacti are not meant to last anyway and they are well known for their habit of dying suddenly, especially in winter.
That looks terminal to me. If you have another cactus you can use as a rootstock you could try your hand at grafting the moon cactus, which still looks healthy in the photo. Moon cactus will not survive by themselves as they lack chlorophyll. Here is a really good page on moon cactus: Moon cactus – Laidback Gardener
