The plant in the first pic looks like Cereus peruvianus, the whitish coat is normal on older part of the plant as sun protection. It builds up over time and goes away too. Don't rub it off, the plant looks good.

The second plant looks like a San Pedro and there are many different possible causes for it to turn yellow. Too much or too little water, too much or too little sun, and diseases. The weird thing here is that the old stems look perfectly healthy. Is it one plant or 2 close to each other and how do they feel to the touch, squishy, hard/dry, wrinkly? My first guess given the drought in CA is too little water. Although a cactus, San Pedro do need quite a bit of water and drought could affect the younger stems first. But maybe I am wrong...

First one is a Cereus, like the previous commenter said and it's fine, looks good.

The second is a San Pedro and it's interesting. It could be from drought or it could be from a nutrient deficiency. But tbh, the yellow color looks a lot like the variegation I see from some specialty SP growers. Spontaneous variegation happens, not usually on a scale like this where the new arms come up completely variegated, but it's not impossible. Did the two yellow arms come up colored yellow? Are there any more pups at the base coming up yellow? If the yellow seema to be spreading from the current arms colores yellow to the older growth, then it is not variegation, but something else.

Could you take more pictures from a bit closer?

I will take some photos today. One arm yellow arm died. I don't remember seeing any new pups.

Ah, that is unfortunate to hear. Perhaps we can help you narrow down the cause of the yellowing.