Acan suddenly looking bad

[Hilltopreef90]

I noticed yesterday that two of my acans were suddenly looking like they were dying on one one of the heads were about gone and the other was receding in and the color was off around the outer edges, this happened in less then two days and there’s been no major change to my tank.
Just in case it’s a lighting issue I moved them into an area that’s more shaded, and I fed them with reef roids late last night hoping that might help
Can they take a turn that fast and could light cause the problem?
My urchin was crawling over another coral close by and may have crawled over the acan as well, would that cause one head to almost completely recede?
any ideas on causes and ways to help nurse them back to health?

 

[G Santana]

IME when nitrates start to rise my acans will let me know immediately.
Once they are under control, acans are happy. Regarding light, I keep mine near the bottom of the tank, I noticed mine losing color higher and are happier down low

 

[Hilltopreef90]

I have 3 other acans in the same area that look fine, my nitrates are high and hopefully a water change will help. I just don’t know why just the one went from being plump and healthy to looking like it’s dying and why only the one.