This plant is either underwatered(not likely) or rotting. Try poking the wrinkled part to see if it is soft. If its rotting you will need to chop off the top and root it

Thanks for the quick reply, Rhamel. The base is not soft to touch. It is also about the same color as the rest of the plant.

Just wondering about so much light:..."The lights are on about 20 hours per day and some days LED are left on round the clock as I often forget to turn off the light at night"... Plants need some sleep, just as ppl do...12-14 hrs/day should be sufficient.

True, i might have gone overboard on the hours. LOL I got very excited after my NOID aeonium which I assumed was an A arboreum turned from light green to a very dark purple-red color, now I'm suspecting might either be a velour or a zwartkop (pic below). My Kiwi after becoming too leggy and bland dull green showing a tricolor look again. I kept the lights on mostly to keep my Vera Higgins and California sunset looking vibrant and the rosettes compact until I can safely move them out again. I cannot wait to get warm again.

Same here, waiting for weather to warm up enough to put plants outdoors - but it will be probably at least 2.5months here...In the meantime, do not 'kill your plants with kindness'...They won't be getting 20hrs of light outdoors (well, depending on where one lives :).

Appropriate hours of good light will do the same. Let them sleep too...

Im not sure what to think

Is the light truly purple, or is it just the camera? The spectrum lookls off-color and very different from sunlight. I've seen a documentary many years ago (Don't recall what program was that), where plants cultivated under light for 24 hours a day actually stopped growing. I don't know the severity of that effect, but depending on lattitude, seasonal change in illumination duration is a part of their life cycle.

"This happened to be the side facing the light...": Was that the same for all cacti or just this one? If not, then do inspect the roots. It may shed some light on the problem if you unpot it and inspect the root ball. Sometimes a response to stress may be addressed simply by replacing the soil. Sometimes there is a root rot or other infection.

It is purple under LED light. This is actually in my basement. My windows do not get enough sunlight this time of the year. The long hours are actually to compensate for the plants under that shelf which does not get much light the pic shows the top level tier, there's two more levels below that. The bottom shelf do not have its own LED panel and relies on the diffuse light coming from the top two shelves. So far, the plants are growing. I'm very happy with the set-up. Truth is, I'm not worried about the cactus, it didn't look like it was dying. I was simply curious. The other cactus plants were doing well, the Mamillaria zeilmanniana continues to bloom.

Kara, those blooms are lovely. Cant wait for mine to bloom.

"I had one that I kept putting off and putting off. Until it pretty much burst out the pot and planted itself" - I laughed so hard reading this.

Hey Chris I was thinking about your wrinkled Trichocereus the other day. How's it doing;)?

He’s getting bigger, Kara. I will be reporting it this spring. LEGO block next to it for scale.