A few days ago, I heard a loud "pop!" but couldn't find the source. Since then, some random corals in my tank have not been doing well, and I couldn't figure out why since others were perfectly fine. I found the source.

I replaced this bulb with a new 400w XM bulb, and hope the corals will be able to recover. What happened is when the glass jacket cracked, that line of UV was hitting the corals.  They were shut down, and growing paler by the day.  The new bulb quickly corrected the problem, and the corals recovered. I'm glad I spotted this in time.

It's always something.