My 12" Deresa spawned 3 months in a row, almost on exactly the same day, almost exactly same time (an hr or so before lights out) as shown by what was documented on a calender.
Water conditions were good according to Salifert tests (KH 8.6, Ca 450, PH 8.3, and nitrates 5ppm or lower), but the mantle would pull in, all of a sudden to shoot out a cloudy jet of eggs/or sperm which the Kole tang was all about taking advantage of though I'm not too sure exactly how he was eating it since it was just a milky cloud to my eyes.
It's only a 75g, and the first time it happened, freaking out I quickly readied a 30g water change (luckily I've always got a rubbermaid trashcan filled with RODI for top off) in case it would be needed, but keeping a close eye on it that eve. (taking some great pics) and all night with a flashlight, the SPS and few LPS seemed to have loved it, both before and after lights out, so I just "let it be" to see what would happen since obviously the whole tank was feeding heavily off it.
Next morning everything looked better than ever, including the Deresa, 2 Croceas, and corals though I went ahead and did the water change so the skimmer wouldn't have to work any more than it already was.
Due to work, I wasn't ever home again anywhere near lights out so never knew if it continued it's monthly spawn or not.
I will add this though, both before and after the spawn, the D. gave no indication whatsoever ("closed for the last week" like you described your C. doing). It just would close up during the duration of the spawn itself, looking awesome as ever right before, and just the same the next morning like nothing had ever happened.
