Help with receding candy cane coral

[edikpok]

Happy New Year everyone!

Among a variety of other corals, I have 3 colonies (30 heads+) of candy canes. Over the past week, many candy canes in these colonies started receding and now each colony has 4-6 canes that almost completely lost their flesh. Some of the other canes started showing their white backbones too... It looks like the receding starts from the base of each cane. The colonies are not located next to each other...Other corals appear to be fine.

Parameters are 1.024 for salt (IO), 0 for nitrates, 380 calcium, 9 KH, and phosphates are at 0.1. It is a 29g biocube, with weekly water changes of 20%. Tank has been established for over 5 years with great growth.

What can possibly cause these canes lose their flesh and die?

Thanks for the help!!!