My pencil cactus is starting to shrivel up, the newer growth is thinning, and im wondering why. I have a second pencil cactus with a heartier stalk that is doing fine - but this one, which has been healthy since I purchased it about eight months ago, is thinning.

Typically I give it a sip of water every couple of weeks. Around 1/4 or 1/3 cup. It’s gotten a bit cooler so I’ve held off watering it on days where the weather is in the lower 60s. It gets plenty of natural light and I do turn on a red blue plant led light on cloudy New England days.

Am I watering it too little at a time? Should I give it more water when I do water it?