[Van down by the river]

I know it's a little late to reply, but anyways, here goes

I noticed Nullig mentioned his pulse in the morning. I believe this may be caused by a day to night PH swing. As the Ph is rising in the morning it reaches a threshold that"triggers" the Xenia. Once the PH rises above this target zone, the pulsing stops. Just a theory. It reminded me of a section in Anthony Calfo's book:

"Average seawater has a pH of approximately 8.4 and every attempt should be made to maintain seawater within tenths of a point of this target. Some species are quite sensitive to declining pH. I recall watching colonies of Xenia in my greenhouse, numbering more than one thousand individuals, stop pulsing en masse when the pH of the system fell below 8.3 and promptly resume pulsing when I corrected it. I do not mean to say that the pulsing of Xenia is only related to pH levels because I do not believe that to be true. However, the imposed stress of a depressed pH is certainly one catalyst that appointed the cessation of pulsing Xenia. It was quite a sight to see and very predictable as I ran digital pH meters on Xenia grow out systems that would practically announce the event with an audible alarm when the pH fell to a low point on the program."- Anthony Calfo

If your Xenia cease again in the future it would be interesting to see the results of numerous pH tests through out the day/night to see the correlation.
Anyways, with any luck it was just a one time thing.