Another pic.

I have the same problem with a Selenicereus pteranthus. Two years old, getting those crusty discolorations around the joints. Mine is tied to dowels, but I have not noticed any leaning or lack of vigor in the plant. Only the uppermost portions of the plant seem affected.

Where to begin? Obviously not nearly enough light, as it is severely etiolated, with that long spindly growth due to insufficient lighting. Also the soil is far too organic, and water retentive. Most Cacti prefer a free draining potting medium. Also the pot is too large for the plant.

Google is your friend, please remember that.

Christopher

@Kaktuskris
Thanks for the info! I have been doing some research on google (that's actually what lead me here), but I figured it couldn't hurt to ask in this forum. Finding it a bit hard to get some solid info, everyone seems to have a different opinion on the how-to's...and I'm not exactly sure what kind of cactus I have. I'll transplant into a smaller pot, but I thought I was using the proper soil?
Is this stuff no good?

Sadly, this soil is not great for succulents. I don't grow cacti, but my succulents all had problems in this soil. Kind of makes you mad when you realize that you really didn't find the perfect soil after all, huh?

You need to add at least 50-60% perlite (or some other gritty substance) to give adequate drainage and "wiggle room" for the roots. My plants seemed fine for about a year, then the soil was compacted around the roots and not even absorbing water any longer. Your plant will appreciate a grittier mix.

You can search on this forum for soil mixes, gritty mixes. But for the immediate remedy I recommend adding the perlite to your soil. Regular sand is not recommended, but some use the coarse builder's sand for grit.

Well you may have had it for just a while but the base looks very old. Can't imagine how many plants that thing has produced.

Opuntia (of some type) with top thinner looking growth, seems to be a little etoliated.
Etoliatiation happens when..... To much water to often or to frequently watered with a sever lack of brightness.

Your plant is ..... Hmmmmm, well I'd wack and trash the thinner top growth and with the bottom start a new in an open pourus mix.

smaller pot indeed. the soil police are gonna get you guys.