First your cactus is a Selenicereus testudo aka Dog-Tail cactus. In case you didn’t know. It’s always best to know whatcha got;).

Next I personally find this cactus hard to grow because my environment is not the best for it. They’re jungle type cactus. So they need a little more water and warmer conditions. My dry hot summers and cool wet winters are not ideal. I bought a big pot from Walmart one year and by the end of that years’ winter it was destroyed, orange spots all over. I didn’t do my research. Looking back I’m pretty sure it was fungus and a little bit of frost damage. I’m saying this because we all learn from each others mistakes;).

Could you tell me how yours is grown?

Thank you! I never did know what it was!

I grow mine inside on a southwest-facing windowsill, so fairly consistent temperature around 70 degrees and decent sun. It is somewhat near a vent, so over the summer it's been a couple feet away from A/C air, but definitely not in the flow of air itself. I water it about every 1.5 weeks.

Thank you! That's very helpful and reassuring. Just in case the scale of the spots matters, there are probably twelve or more spread over the various branches of the cactus, with about six on just one branch. Are you sure they aren't some kind of canker? I found this photo what they called "cactus canker" when I was looking for different types of cactus disease - it had me worried because of how similar it looks to what my cactus seems to be developing:

If not, any other tips for how to make my cactus's living environment more hospitable? I unfortunately can't take it outside because I live in an apartment, plus winter is just around the corner... Does it like to be misted?

Nick, some cactus (not the one you have pictured immediately above) develop corking, a perfectly natural occurrence where the lower epidermis becomes scabbed-like, but it's no actual harm to the plant. I suspect that what's happening there is more culturally-based, similar to your plant.

No, I'd not suggest misting, which is great for rootless cuttings, but fairly ineffective for established plants. Is it possible that you can increase available water by making the soil more porous (which means you can water maybe 1x a week)? Other than making the whole room more humid, that's all I've got. And with a more humid room, you'd want more air circulation.

This cactus is not all that much seen in nurseries.

Demea testudo, by whatever name, is so susceptible to Orange Rust that I quit growing it . Of all the epiphytes I've ever had, Demea is the only plant that ever died from it. Large plants like Epiphyllums get holes in the leaves, but Deameas actually die. Warm and dry is the only cure. This may be the problem, but I hope it hasn't visited you.

Also, this plant grown indoors rarely blooms. Don't know why. Grown outdoors in Heaven (San D), it blooms as a small plant. Also, as The Dean says, your Ferocactus has corking. Ugly to some humans, but not harmful to the plant. Feros and Opuntias seem to revel in corking. Think of it as making bark.

Edit: Missed this in McHarris's post, air circulation. As important as light. If it is in still air, get a small computer fan and run it 24/7.