I wouldn't trim it. Just read it as a sign that it is very stressed, and if something doesn't change, it will die. Perhaps moving it to a different location in the tank, such as towards the top, could make it happier.
To say that nothing changed might not be entirely accurate. Corals thrive on consistency, so if it has done well in the past, and it is not doing well now, is a sign that something has likely changed. That exact parameter is probably something that you haven't tested for yet, that's all.
Hope that helps?
Jeff