I’ve kept Duncan’s as long as I’ve been in the hobby.
I’ve seen the tentacles super long and flowing in the breeze like the hair of a guy driving a convertible in a Levitra commercial.
I’ve also seen them bubble like an anemone, curl up like you’re seeing, and even completely close at seemingly random intervals.
I did recently lose a huge colony to brown jelly disease, which was really surprising as it had been indestructible for years.
keep an eye on it, but they do change their morphology a lot in my experience.
My Duncan is looking exactly the same.. It used to be much more extended and looked happy, now its looking shrivvled and starting to worry. I fed it Fauna Marin Coral dust, as much as it enjoys it, it remains looking like in the photo above..
I notice that mine do that at varying times. Usually it’s because of my hermits. There are 13 in my system which is a 90 gallon cube.
Even though your duncans are a bit shriveled are they showing signs of growth? My Duncans are growing two new polyps currently. Even so I still see mine looking like yours every now and again.
I know that sudden changes in water quality/chemistry can mess with corals in general. Have there been any changes in your system or new fish or inverts added recently?
Edit:
I just realized this is an antique thread. Hopefully your duncans are doing well now.
Im not the OP but I replied because I have the same issue. Yes I was seeing fantastic growth and I had 2 small polyps growing and now its all shrivelled. As mentioned target feeding Fauna Marin Coral dust didnt make a difference. Will try spot feed mysis today. Will check parameters when back from work as yest I did a water change